Abstract
Changes of molecular spin are ubiquitous in chemistry and biology. Among spin flip processes, one of the fastest is intersystem crossing (ISC) in transition metal complexes. Here, we investigate the spin relaxation dynamics and emission spectrum of [Re(CO)3(im)(phen)]+ (im = imidazole, phen = phenanthroline) using extensive full-dimensional excited-state dynamics simulations in explicit aqueous solution. Contrary to what has been observed in other transition metal complexes, the transition from the singlet to triplet states occurs via a two-step process, with clearly separable electronic and nuclear-driven components with two different time scales. The initially excited electronic wave function is a "molecular spin-orbit wave packet" that evolves almost instantaneously, with an 8 fs time constant, into an approximate 25 : 75 singlet-to-triplet equilibrium. Surprisingly, this ISC process is an order of magnitude faster than it was previously documented for this and other rhenium(i) carbonyl diimine complexes from emission spectra. Simulations including explicit laser field interactions evidence that few-cycle UV laser pulses are required to follow the creation and evolution of such molecular spin-orbit wave packets. The analysis of the dynamics also reveals a retarded ISC component, with a time constant of 420 fs, which can be explained invoking intramolecular vibrational energy redistribution. The emission spectrum is shown to be characterized by ISC convoluted with internal conversion and vibrational relaxation. These results provide fundamental understanding of ultrafast intersystem crossing in transition metal complexes.
Highlights
Contrary to what has been observed in other transition metal complexes, the transition from the singlet to triplet states occurs via a two-step process, with clearly separable electronic and nuclear-driven components with two different time scales
The initially excited electronic wave function is a “molecular spin–orbit wave packet” that evolves almost instantaneously, with an 8 fs time constant, into an approximate 25 : 75 singlet-to-triplet equilibrium. This intersystem crossing (ISC) process is an order of magnitude faster than it was previously documented for this and other rhenium(I) carbonyl diimine complexes from emission spectra
A er vertical excitation the electronic wave function is not anymore an eigenstate of the total Hamiltonian; due to the strong spin–orbit couplings it can be best described as a linear combination of spin–orbit eigenstates. Such a linear combination—that might be called a “molecular spin–orbit wave packet”—undergoes nontrivial time evolution, which manifests itself in ISC that occurs almost instantaneously
Summary
Rhenium(I) carbonyl diimine complexes—convenient photosensitizers for studying long-range electron-transfer in proteins14,15—are another example of complexes where the detailed ISC mechanism remains elusive. ISC, which facilitates a transition from the initially excited singlet metal-to-ligand charge transfer (1MLCT) states[16,17,18] to several triplet states[16,19,20,21,22,23,24] is reported on a 100 fs time scale, and slower than in other complexes containing Ru or Fe
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