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)3Cl] 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 (1LC), the triplet metal-to-ligand charge-transfer (3MLCT), and the triplet ligand-centered (3LC) 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 3MLCT and 3LC excited states present in Re-H is modulated in favor of the 3LC 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.