Theoretical probe of absorption and fluorescence emission characteristics of highly luminescent ReL(CO)₃X (L = 12H-indazolo[5,6-f][1,10]phenanthroline and X = F, Cl, Br, I): a DFT/TD-DFT study

Four novel rhenium (Re) tricarbonyl complexes (ReL(CO)₃X; X = F, Cl, Br, I and L = 12H-indazolo[5,6-f][1,10]phenanthroline) with promising luminescent properties were investigated using a DFT/TD-DFT approach at the ωB97X-D3/6-31 + G(d,p)//LANL2DZ level of theory. The effects of halogen co-ligands (F, Cl, Br and I) and solvents (acetonitrile (ACN), dichloromethane (DCM) and dimethylsulfoxide (DMSO)) on luminescent properties of the complexes were examined. The studied Re(I) tricarbonyl complexes assumed distorted octahedral geometry with an N–Re–N bite angle of ca. 75°, and showed possible p–π interactions between the halogen (X) atom and the N-donor ligand (L) ring. Prominent electronic excitations (with high oscillator strengths) are S₀→S₃, attributed to the hole–electron orbital transition based on the natural transition orbitals analyses, and characterised as MLCT/LMCT. The complexes exhibited prominent Stokes fluorescence with Stokes shifts ranging from 9204 to 19490 cm⁻¹. ReL(CO)₃Cl in ACN showed the largest Stokes shift, whereas ReL(CO)₃I in ACN displayed the smallest Stokes shift. Fluorescence emission wavelengths varied with X and solvent, though the four complexes emit in the cyan/blue range in DCM. ReL(CO)₃F (in ACN) exhibited the highest fluorescence lifetime and quantum yield. The study demonstrates the tunability of photophysical properties and promotes the theoretical probe of optical characteristics of materials as corroborative efforts to experimental findings.