Excited State Dynamics of a PtII Diimine Complex bearing a Naphthalene-Diimide Electron Acceptor

A combination of picosecond time-resolved infrared spectroscopy, picosecond transient absorption spectroscopy, and nanosecond flash photolysis was used to elucidate the nature and dynamics of a manifold of the lowest excited states in Pt(phen-NDI)Cl2 (1), where NDI = strongly electron accepting 1,4,5,8-naphthalene-diimide group. 1 is the first example of a PtII-diimine-diimide dyad. UV/vis/IR spectroelectrochemistry and EPR studies of electrochemically generated anions confirmed that the lowest unoccupied molecular orbital (LUMO) in this system is localized on the NDI acceptor group. The lowest allowed electronic transition in Pt(phen-NDI)Cl2 is charge-transfer-to-diimine of a largely Pt→phen metal-to-ligand charge-transfer (MLCT) character. Excitation of 1 in the 355−395 nm range initiates a series of processes which involve excited states with the lifetimes of 0.9 ps (1NDI*), 3 ps (3MLCT), 19 ps (vibrational cooling of “hot” 3NDI and of “hot” NDI ground state), and 520 μs (3NDI). Excitation of 1 with 395 nm femtosecond laser pulses populates independently the 1MLCT and the 1NDI* excited states. A thermodynamically possible decay of the initially populated 1MLCT to the charge-transfer-to-NDI excited state, [PtIII(phen-NDI−•)Cl2], is not observed. This finding could be explained by an ultrafast ISC of the 1MLCT to the 3MLCT state which lies about 0.4 eV lower in energy than [PtIII(phen-NDI−•)Cl2]. The predominant decay pathway of the 3MLCT is a back electron transfer process with ∼3 ps lifetime, which also causes partial population of the vibrationally hot ground state of the NDI fragment. The decay of the 1NDI* state in 1 populates vibrationally hot ground state of the NDI, as well as vibrationally hot 3NDI. The latter relaxes to form 3NDI state, that is, [Pt(phen-3NDI)Cl2]*, which possesses a remarkably long lifetime for a PtII complex in fluid solution of 520 μs. The IR signature of this excited state includes the ν(CO) bands at 1607 and 1647 cm−1, which are shifted considerably to lower energies if compared to their ground-state counterparts. The assignment of the vibrational bands is supported by the density-functional theory calculations in CH2Cl2. Pt(phen-NDI)Cl2 acts as a modest photosensitizer of singlet oxygen.