Excited-State Proton Transfer Controls Irreversibility of Photoisomerization in Mononuclear Ruthenium(II) Monoaquo Complexes: A DFT Study.

The detailed DFT investigation clears the working mechanism of the irreversible photoisomerization of trans-Ru(tpy)(pynp)(OH2) (TA) and cis-Ru(tpy)(pynp)(OH2) (CA) complexes. Both TA and CA complexes present two types of low lying triplet states, one resulting from a triplet metal-ligand charge-transfer (TMLCT) and the other from a triplet metal-centered d-d transition (TMC). The vertical excitation of the singlet ground state of the complexes leads to a singlet excited state, which undergoes ultrafast decay to the corresponding TMLCT. For TA, this TMLCT transforms with a low barrier to a TMC state. The dissociative nature of the TMC state leads to easy water removal to produce a five-coordinate intermediate that can isomerize via rotation of a pynp ligand and proceed towards the CA product. For CA, however, during this excitation and intersystem crossing process, an excited-state proton transfer (ESPT) occurs and the resultant TMLCT is very much stabilized with a very strong Ru(II)-OH bond; the high barrier from this TMLCT blocks conversion to a TMC state and thus prevents isomerization from the cis to the trans isomer. This high barrier also prevents the possibility of the isomerization process from TA to CA solely on the adiabatic triplet pathway. Instead, crossing points (XMC-CB, XMC-CA) near the minimum of the triplet metal-centered state of the cis isomer provide nonadiabatic decay channels to the ground-state S0--CA, which completes the photoisomerization pathway from TA to CA.