Palani Elumalai

Luminescent Dirhenium(I)-Double-Heterostranded Helicate and Mesocate

The semirigid ligands 1,4-bis(2-(2-hydroxyphenyl)benzimidazol-1-ylmethyl)benzene (H2-pBC) and 1,3-bis(2-(2-hydroxyphenyl)benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene (H2-mBC), containing two hydroxyphenylbenzimidazolyl units as bis-chelating (or bis(bidentate)) N∩OH donor, were synthesized and were used to assemble neutral, luminescent heteroleptic, unsaturated double-hetero-stranded, rhenium(I)-based helicate (1) and mesocate (2) with the flexible bis(monodentate) nitrogen donor (1,4-bis(benzimidazol-1-ylmethyl)benzene/1,3-bis(benzimidazol-1-ylmethyl)benzene), and Re2(CO)10. The photophysical properties of the complexes were studied. Both complexes 1 and 2 exhibit dual emissions in both solution and solid state. In solution, these complexes show both fluorescence and phosphorescence. Complex 1 undergoes a predominantly ligand-centered oxidation, resulting in the generation of phenoxyl radicals.

New class of phosphine oxide donor-based supramolecular coordination complexes from an in situ phosphine oxidation reaction or phosphine oxide ligands.

A one-pot, multicomponent, coordination-driven self-assembly approach was used to synthesize the first examples of neutral bridging phosphine oxide donor-based supramolecular coordination complexes. The complexes were self-assembled from a fac-Re(CO)3 acceptor, an anionic bridging O donor, and a neutral soft phosphine or hard phosphine oxide donor.

Design strategy for arranging an aromatic cyclic trimer into a tripodal molecule

Arranging an aromatic cyclic trimer into a tripodal molecule was achieved by the reaction of tri(bromomethyl)benzene with substituted benzimidazoles. The edge-to-face C–H⋯π interactions in the trimer stabilize the conformation of flexible molecules.

Exclusive excited state intramolecular proton transfer from a 2-(2′-hydroxyphenyl)benzimidazole derivative

The excited state intramolecular proton transfer (ESIPT) of a newly designed 2-(2′-hydroxyphenyl)benzimidazole derivative (bis-HPBI), has been investigated in different nonpolar, polar aprotic, and polar protic solvents. Unlike 2-(2′-hydroxyphenyl)benzimidazole, it exhibits exclusive ESIPT even in protic solvents. The existence of trans enol was made unviable by crafting a steric hindrance that stops the normal emission of bis-HPBI. Though bis-HPBI has two HPBI units, the tautomer emission of bis-HPBI is due to only single proton transfer. The experimental studies and theoretical calculations corroborate the finding. Protonation and deprotonation studies on bis-HPBI are also performed. The enhancement in the tautomer band intensity upon deprotonation of one of the OH groups also supports the single proton transfer in bis-HPBI. On the other hand, the initial addition of acid quenches the tautomer emission by hydrogen bonding interactions. After protonation of imidazole nitrogen, bis-HPBI acts as a photoacid. The dissociation of ‘OH’ protons and reorganization of the molecule leads to partial recovery of tautomer emission. In a strongly acidic medium where deprotonation of the ‘OH’ group is not possible, the emission is observed from the cation of bis-HPBI.

Rhenium(I)-Based Monocyclic and Bicyclic Phosphine Oxide-Coordinated Supramolecular Complexes

Neutral, flexible ditopic phosphine (P–P) or phosphine oxide (O=P–P=O) donors, rigid anionic bis-chelating oxygen donors, and Re2(CO)10 were used to assemble ten phosphine oxide (P=O)-donor-based neutral monocyclic M2LL′-, bicyclic M4L2L″-, and bicyclic M4LL′2-type supramolecular coordination complexes (SCCs). A soft ditopic phosphine donor was transformed into a hard ditopic phosphine oxide donor, during the formation of the cyclic complexes 1–3, 5–6, and 9–10. Complexes 4, 7, and 8 were obtained using a hard P=O donor ligand. These SCCs were characterized using elemental analysis, FTIR, NMR, and single-crystal X-ray diffraction analysis. The absorption properties of 1–8 were studied using absorption UV–vis spectroscopic methods, and the results were analyzed using theoretical calculations. The results revealed that the neutral P=O donor significantly influenced the photophysical properties by enhancing the absorption coefficient in the visible region.