Nature and Dynamics of the Excited States of Ethynyl-Functionalized Naphthalimide–Phenanthroline Re(I) Complexes Studied by TA and TRIR Spectroscopies

The synthesis and photophysical properties of four [Re(R–NI–phen)(CO) 3 Cl] complexes ( Re–R ), where NI = 1,8-naphthalimide and phen = 1,10-phenanthroline, are reported. The R–NI chromophores were substituted at their 4-positions with R = H ( Re–H ), ethynyl benzene ( Re–EB ), ethynyl thiophene ( Re–ES ), ethynyl trimethylsilane ( Re–ET ), yielding a series of metal-organic bichromophores. The R–NI–phen organic chromophores were investigated in parallel to better understand the photophysical properties exhibited in the Re–R complexes. The excited-state processes of these molecules were studied by transient absorption and time-resolved infrared spectroscopies, to unravel the kinetics of the energy transfer processes occurring in the Re–R series. Namely, the interplay between the singlet ligand-centered ( 1 LC), the triplet metal-to-ligand charge-transfer ( 3 MLCT), and the triplet ligand-centered ( 3 LC) excited states were studied using these time-resolved spectroscopic techniques, supported by quantum chemical calculations, to monitor the changing excited-state dynamics in the Re–R series. These studies reveal that the thermal equilibrium between the 3 MLCT and 3 LC excited states present in Re–H is modulated in favor of the 3 LC excited state in the remaining Re–R series, giving rise to increasingly ligand-like photophysics as the electron-donating strength of the R-group increases.