B-N Lewis pair-functionalized dihydrophenazines

B-N Lewis pair functionalization of polycyclic aromatic hydrocarbons (PAHs) has attracted attention due to their promise for applications in organic light emitting diodes (OLEDs), solar cells, sensory and biological imaging materials, as photoresponsive materials, and in organic field effect transistors (OFETs). In B-N Lewis pairs B accepts the lone pair from the nitrogen. Borylation of PAHs containing N-heterocyclic substituents can lead to an extended coplanar scaffold encompassing the PAH core and N-heterocyclic substituents which alters the energy levels, thus generating unusual optical and electronic properties. This thesis focuses on the synthesis and properties of B-N Lewis pair functionalized dihydrophenazines. In Chapter 2, the structure-property relationships of B-N Lewis pair complexes derived from ortho-pyridyl substituted N,N-dihydrophenazine without, with one and two fused benzene rings were studied in detail. Borylation of these N,N-dipyridyl-N,N-dihydrophenazines with BBr3 in the presence of tertiary amines as base led to doubly B-N fused complexes with both boron centers attached to the same benzene ring. The products showed a large red shift in emission compared to the free ligands due to lowering of the LUMO energies. The B-N Lewis pair functionalization also rigidifies the dihydrophenazines as it hinders the rotation of the N-heterocyclic substituents. Due to this, the B-N Lewis pair functionalized dihydrophenazines showed aggregation-induced emission enhancement (AIEE) effects in their aggregate form. We studied their structural features in the solid state by X-ray diffraction (XRD) analysis and examined the electronic properties by electrochemical measurements and density functional theory (DFT) calculations. Nucleus-independent chemical shift (NICS) calculations were performed to assess the aromaticity of the central dihydrophenazine ring, showing that after borylation the antiaromatic character decreased due to changes in the electronic delocalization and quinoidal characteristic observed after B-N Lewis pair formation. In Chapter 3, we studied the effect of replacing ethyl with phenyl substituents on B in the B-N Lewis pair functionalized dihydrophenazines. We found that by changing the substituents to Ph makes the B center more electron deficient, resulting in relatively lower LUMO energy of the phenyl compared to the ethyl derivative. The reduction potentials became less negative, and oxidation potentials became more positive due to the electron withdrawing effect of the Ph groups. The Ph groups also increased the overall rigidity of the system which resulted in higher fluorescence quantum yields compared to the ethyl derivatives. The ethyl- and phenyl-substituted B-N Lewis pair functionalized dihydrophenazines were studied as photocatalysts for organo-catalyzed atom transfer radical polymerization (O-ATRP) of methyl methacrylate (MMA) to generate poly(methyl methacrylate) (PMMA). The ethyl derivatives promoted the polymerization without an external initiator whereas the phenyl derivatives required addition of an external initiator, suggesting that ethyl radicals generated by B-C bond cleavage in the presence of light might be acting as radical initiator.In Chapter 4, we explored mechanistic aspects of the borylation of dihydrophenazines in an effort at shining light on the preferential formation of cis-isomers (double borylation on the same phenyl ring) over trans-isomers (borylation on different phenyl rings). Experimental studies on the borylation of N,N-dipyridyl-N,N-dihydrophenazine with BBr3 and DIPEA as base at 110 °C showed that the cis-isomer is generated as major component and the trans-isomer as minor component in a ca. 10:1 ratio. We performed computational studies to gain insights into reasons for the preferential formation of the cis-isomer. The calculations showed that the ground state energy of the trans-isomer is lower than that of the cis-isomer, suggesting that it is the thermodynamically more favorable product. However, the Gibb’s free energy for the transition state involved in the formation of the cis-isomer was found to be lower than for the trans-isomer, which implies that the cis-isomer is preferentially formed under kinetic control. Electrochemical analyses showed that the trans-isomer is more difficult to oxidize and more difficult to reduce than the cis-isomer, suggesting a higher lying LUMO and lower lying HOMO, this is also reproduced by computation. The absorption maximm (λmax) of trans isomer is more blue shifted than absorption maximum (λmax) of trans isomer. The cis-isomer showed slightly red shifted emission than trans isomer. The trans-isomer showed higher quantum yield than cis-isomer. The trans-isomer has shorter lifetime than cis-isomer.