Triarylborane Functionalized Dicyanovinyl and Acetylacetone Based Molecular Platforms: Building Blocks for Multiple Anion Sensors and Efficient Phosphorescence Emitters The main objective of this thesis is to design a simple strategy for triarylborane based multiple anion sensors and development of triarylborane incorporated phosphorescent metal complexes. The thesis consists of eight chapters and the contents of each chapter are given below. Chapter 1 This chapter gives a general introduction to recent advances relevant to the theme of the thesis. A review of the fundamental characteristics of triarylboranes and their applications in various fields such as chemical sensors and optoelectronics is presented. Advances in boron chemistry in the areas such as anion sensors, solid state emissive and phosphorescence materials are discussed in detail. The scope of the thesis is outlined at the end of the chapter. Chapter 2 The second chapter deals with the general experimental techniques and synthetic procedures followed in this thesis. Chapter 3 This chapter deals with a rational design strategy for differential identification of fluoride and cyanide ions using TAB based sensors. In general, most of the triarylboranes give similar optical responses towards fluoride and cyanide ions as they follow similar sensing mechanism. In order to circumvent this problem, two TAB-DCV conjugates (1 and 2) are designed and synthesised. The DCV unit is highly specific for cyanide ion owing to the presence of electrophilic carbon center. Probes 1 and 2 differ in steric crowding around the boron center. The less crowded boron center in 1 binds with fluoride as well as with cyanide ions giving similar optical response (luminescence is quenched in presence of F¯ and CN¯). In the case of 2, selectivity of boron center towards fluoride is tuned by increasing the steric crowding around the boron unit. The dicyanovinyl unit acts as selective sensing site for cyanide ions. As a result, 2 gives different fluorogenic response towards the anions F¯ and CN¯ which were considered as interfering anions in TAB based sensor chemistry. Thus, a modular design principle is developed for differential identification of fluoride and cyanide ions using TAB. Chapter 4 In this chapter, detailed photophysical studies of TAB-amine-DCV conjugates and colorimetric discrimination of fluoride and cyanide ions are discussed. Presence of amine based donor between the two electron deficient sites enhances the electronic conjugation in 3−5. Since there are two different acceptor sites with a common donor, two distinct charge transfer transition bands are observed in the visible region of electromagnetic spectrum. The absorption and emission spectra of these compounds show pronounced sensitivity to solvent polarity, signifying large excited state dipolmonents. Anion binding studies confirms that these compounds are highly selective towards fluoride and cyanide ions. Fluoride ions selectively interact with boron center and block the corresponding charge transfer transition thereby leading to a distinct colour change which is observable by naked eye. On the other hand, cyanide interacts with boron as well as DCV unit and blocks both the charge transfer transitions which results in disappearance of colour. Hence, compounds 4 and 5 exhibit different colorimetric signals for fluoride and cyanide ions. Since the absorption bands of 3 do not fall in the visible region, it does not show any colorimetric response towards the aforementioned anions. The anion sensing mechanisms are established by 1H, and 19F NMR studies. Chapter 5 This chapter presents a systematic study of the effect of length of π-electronic conjugation on the optical properties and anion sensing abilities of a series of TAB-oligothiophene-DCV conjugates (6−8). Their absorption as well as emission bands undergo redshift upon increasing the number of thiophene units between TAB and DCV units as the π-electronic conjugation in 6−8 is greatly dependent on the number of thiophene units. Their fluorescence emission is highly sensitive to solvent polarity. In the case of 6, the emission band undergoes a redshift with reduced intensity. In the case of 7 the emission band undergoes a redshift but the intensity is not affected by solvent polarity. In the case of 8, the emission band undergoes redshift with enhanced intensity in polar solvents. Interestingly, 7 and 8 show solvent viscosity dependent fluorescence. Structural reorganisation is restricted in viscous medium and results in enhanced emission for 7 and 8. Further, these compounds exhibit selective response towards the fluoride and cyanide ions with different colorimetric responses. Test strips made up of probes 7 and 8 have potential application in identifying fluoride and cyanide ions in aqueous medium. Chapter 6 This chapter describes synthesis and optical characterisation of triarylborane incorporated acetylacetone (acacH) ligands (9, 10) and their borondifluoride complexes (11, 12). AcacH ligands and BF2 complexes show solvent dependent emission phenomena due to the involvement of charge transfer transition. Their optical properties are highly dependent on molecular conformations. Complex with duryl spacer (12) exhibits more red shifted emission in polar solvents due to the enhanced charge transfer transition facilitated by twisted rigid geometry. In presence of fluoride and cyanide ions, the borondifluoride complexes are not stable. The anions concomitantly interact with tricoordinate boron as well as acac-BF2 unit to give rise to complex pattern of photoluminescence spectral changes during the titration experiment. The binding pathway and the possible species involved are established with the help of 1H, 19F and 11B NMR spectral studies in presence of the anions. Complexes 11 and 12 act as selective chemodosimetric sensors for fluoride and cyanide ions. Chapter 7 In this chapter, the synthesis and optical characterisations of triarylborane conjugated cyclometalated platinum complexes are discussed. A series of square planar platinum complexes are synthesised with different cyclometalating ligands. Complexes (13−18) exhibit a range of luminescence from green to red in solution as well as in the solid state. Their emission intensities are highly sensitive towards atmospheric oxygen suggesting that they originate from a triplet excited state. A maximum of 85% quantum yield is observed for complex 15 in solution state while complex 14 showed a maximum of 58% quantum yield in solid state. Complexes with rigid molecular conformation (14, 16 and 18) showed higher luminescence quantum yield than those having phenyl spacer (13, 15 and 17). The sterically encumbered boryl (-BMes2) group significantly reduces π-π stacking between the square planar entities. Thus, complexes 13−18 show bright luminescence in solid state compared to model complexes without boryl group. The effect of Lewis acidic boron center on luminescence behaviour is explored by fluoride binding studies. Chapter 8 This chapter is divided into two parts. Part-I describes the synthesis and optical characterisation of triarylborane conjugated cyclometalated iridium complexes (19−24). They are brightly luminescent in solution state with high sensitivity towards atmospheric oxygen. Complex 20 shows a highest quantum yield of 91%. Interestingly, under ambient atmospheric conditions, they exhibit a rare type of dual emission. Life time data suggest that the lower energy emission band originates from cyclometalated iridium based triplet excited state while higher energy emission band originates from boryl ased singlet excited state. Fluoride binding at the boron site results in luminescence quenching; evidently, tri-coordinate boron has a major contribution to the luminescence features of these iridium complexes. Part-II deals with synthesis of triarylborane conjugated pyrazole ligand (25) and its binuclear iridium complexes (26−28) in which two iridium centers are bridged by hydroxo as well as pyrazolato ligands. These binuclear iridium complexes exhibit higher luminescence quantum yield than TAB-acac-Iridium complexes (mononuclear complexes; part I). Binding of fluoride ions at the boron center has a minor impact on their luminescence nature. High sensitivity of their luminescence towards atmospheric oxygen indicates the involvement of triplet excited state in their emission process.

The main thrust of this thesis is the development of new triarylborane containing luminescent molecules as well as utilizing triarylboron center as a receptor for the selective detection of biologically, environmentally and industrially important anions such as fluoride and cyanide in aqueous and non-aqueous solutions. The thesis contains nine chapters. The contents of each chapter are described below. Chapter 1 The first chapter is an introduction to the theme of the thesis and presents a general review on the techniques, theories and photochemistry relevant to the present work with emphasis on photochemistry of triarylboranes and their importance in the field of anion sensor chemistry. A review on various boron based luminophores is also presented. Chapter 2 The second chapter deals with the general experimental techniques and synthetic procedures utilized in this work. Chapter 3 This chapter deals with the synthesis of boryl-BODIPY dyads (1-8) in which triarylborane acts as anion receptor and BODIPY as a signalling unit. The absorption spectra of all boryl-BODIPY dyads shows similar pattern. However, the fluorescence spectra of 1, 2, 3, 6 and 7 shows dual emission bands whereas 4, 5 and 8 exhibit a single emission band. These interesting photophysical properties of boryl-BODIPYs (1-8) depends on the dihedral angle between two chromophores and partial energy transfer from donor (triarylborane) to acceptor (BODIPY) unit. The energy transfer efficiency of compounds 4, 5 and 8 is higher (close to 100%) compared to other series of boryl-BODIPYs (1-3, 6 and 7), due to the orthogonal arrangement of chromophores with high dihedral angles. To better understand photophysical properties and energy transfer process, anion binding studies were carried out since triarylborane acts as receptor for fluoride and cyanide ions. Anion binding studies of boryl-BODIPYs were (1-5) carried out in dichloromethane solutions and using tetrabutylammonium salt of fluoride/cyanide. All boryl-BODIPY dyads (1-5) were sensitive and selective sensor of fluoride, whereas the presence of only excess amounts (20 equv or more amounts) of cyanide made any changes in absorption and emission spectra. Other anions even above 100 eq were unable to cause any change. The quenching efficiency of compounds 4 and 5 was found to be more than that of other boryl-BODIPYs (1 and 3). The binding of fluoride with boryl-BODIPY (1-5) was entirely reversible; addition of BF3•Et2O to the fluoride adducts of compounds (1-5) regenerated the parent compounds. Chapter 4 In chapter 3, it was established that linear boryl-BODIPY dyads (1-8) show dual/single fluorescence bands depending on the dihedral angle between triarylborane and BODIPY unit. This Chapter describes the synthesis of three new “V” shaped boryl-BODIPY dyads (9, 10 and 11) their optical properties, Compound 9-11 are structurally similar differing only in the number of methyl substituents on the BODIPY moiety which were found to play major role in determining their optical behavior. The dyads show rare forms of multiple channel emission characteristics arising from different extents of electronic energy transfer (EET) processes between the two covalently linked fluorescent chromophores (triarylborane and BODIPY units). Owing to the presence of Lewis acidic triarylborane moiety, the dyads function as highly selective and sensitive fluoride sensors with vastly different response behavior. Upon binding of fluoride to the tricoordinate borane centre, dyad 9 shows gradual quenching of its BODIPY dominated emission due to the cessation of (borane to BODIPY) EET process. Dyad 10 shows ratiometric changes in its emission behavior upon addition of fluoride. Dyad 11 forms fluoride induced nanoaggregates which result in fast and effective quenching of its emission intensity upon addition of even small quantities of analyte (i.e. 0.1 equivalent of fluoride). When the solution is allowed to stand, disaggregation of the molecules results in partial recovery of the initial fluorescence bands. Thus, small structural alterations in these three structurally close dyads (9-11) result in exceptionally versatile and unique photophysical behavior and remarkably diverse responses towards a single analyte i.e. fluoride anion. Chapter 5 This chapter deals with intermolecular charge transfer (ICT) process in borane containing donor-acceptor triads and tetrads to realize colorimetric response for small anions such as fluoride and cyanide. Triad 12 and tetrad 13 incorporating –B(Mes)2, BDY (borondipyrromethene), and TPA (triphenylamine) were synthesized. Introduction of two dissimilar acceptors (triarylborane and BODIPY) on a single donor (TPA) resulted in two distinct ICT process (amine to borane and amine to BDY). The absorption and emission properties of new triad and tetrad are highly dependent on individual building units. The nature of electronic communication among the individual fluorophore units has been comprehensively investigated and compared with building units. Compounds 12 and 13 showed chromogenic and fluorogenic response towards small anions such as fluoride and cyanide. Chapter 6 In the previous chapter, it was demonstrated that although triphenylamine-triarylborane-BODIPY donor-acceptor conjugates show colorimetric response towards fluoride and cyanide. They could not distinguish these two interfering anions. To overcome the anion interference peripherally triarylborane decorated porphyrin (14) and its Zn(II) complex (15) were designed and synthesized and this forms the subject matter of this Chapter. Compound 15 contains two different Lewis acidic binding sites (Zn(II) and boron centre). Unlike all previously known triarylborane based sensors, the optical responses of 15 towards fluoride and cyanide are distinctively different thus enabling the discrimination of these two interfering anions. Metalloporphyrin 15 shows a multiple channel fluorogenic response towards fluoride and cyanide and also a selective visual colorimetric response towards cyanide. By comparison with model systems and from detailed photophysical studies on 14 and 15, it was concluded that the preferential binding of fluoride occurs at the peripheral borane moieties resulting in the cessation of the EET (electronic energy transfer) process from triarylborane to porphyrin core and with negligible negative cooperative effects. On the other hand, cyanide binding occurs at the Zn(II) core leading to drastic changes in its absorption behavior which can be followed by the naked eye. Such changes are not observed when the boryl substituent is absent (e.g. tetraphenyl-Zn(II)-porphyrin or TPP). The conjugates 14 and 15 showed reversible binding interaction towards CN and F and they are capable of extracting fluoride from aqueous media. Chapter 7 This Chapter deals with the design of a sensor which can detect fluoride colorimetrically in aqueous medium. Detecting fluoride in aqueous solution is an important area of current research owing to both positive and negative health and environmental effects associated with the fluoride ion. Although numerous fluoride sensors are reported, the colorimetric sensing (visual detection without the need of costly equipment and complicated analytical of fluoride at recommended levels 0.7 ppm) has not realized. Here we report the design, optical and fluoride sensing ability of two new water soluble Lewis acidic triarylborane-triarylamine conjugates 16 and 17 (containing one or two ammonium cations (-C6H4-NMe3). Compound 17 shows selective colorimetric response for aqueous inorganic fluoride at as low a level as 0.1 ppm Chapter 8 The synthesis and optical properties of four new triarylborane–dipyrromethane (TAB– DPM) conjugates (19a–d) containing dual binding sites (hydrogen bond donor and Lewis acid) have been reported. The new compounds exhibit a selective fluorogenic response towards the F− ion. The NMR titrations show that the fluoride ions bind to the TAB–DPM conjugates via the Lewis acidic triarylborane centre in preference to the hydrogen bond donor (dipyrromethane) units. Chapter 9 A new triarylborane-aza-BODIPY conjugate is reported (22). The conjugate molecule consists of two blue emissive dimesitylarylborane moiety and a NIR (near infra-red) emissive aza-BOIDPY core and shows panchromatic absorption spanning over ~300-800 nm region. The presence of two different fluorophore units in the conjugate leads to a broad dual-emissive feature covering a large part of visible and NIR region. DFT computational studies suggest limited electronic communication between the individual fluorophore units which may be responsible for the intriguing optical features of the conjugate molecule. Further, the broadband emissive conjugate can act as a selective sensor for fluoride anion as a result of fluorescence quenching response in both visible as well as in NIR spectral region.