Ultrafast excited-state dynamics of photoisomerizing complexes fac-[Re(Cl)(CO)3(papy)2] and fac-[Re(papy)(CO)3(bpy)]+ (papy=trans-4-phenylazopyridine)

The character and dynamics of low-lying electronic excited states of the complexes fac-[Re(Cl)(CO)3(papy)2] and fac-[Re(papy)(CO)3(bpy)]+ (papy = trans-4-phenylazopyridine) were investigated using stationary (UV–Vis absorption, resonance Raman) and ultrafast time-resolved (visible, IR absorption) spectroscopic methods. Excitation of [Re(Cl)(CO)3(papy)2] at 400 nm is directed to 1ππ∗(papy) and Re → papy 1MLCT excited states. Ultrafast (≪1.4 ps) intersystem crossing (ISC) to 3nπ∗(papy) follows. Excitation of [Re(papy)(CO)3(bpy)]+ is directed to 1ππ∗(papy), 1MLCT(papy) and 1MLCT(bpy). The states 3nπ∗(papy) and 3MLCT(bpy) are then populated simultaneously in less then 0.8 ps. The 3MLCT(bpy) state decays to 3nπ∗(papy) with a 3 ps time constant. 3nπ∗(papy) is the lowest excited state for both complexes. It undergoes vibrational cooling and partial rotation around the –NN– bond, to form an intermediate with a nonplanar papy ligand in less than 40 ps. This species then undergoes ISC to the ground state potential energy surface, on which the trans and cis isomers are formed by reverse and forward intraligand papy rotation, respectively. This process occurs with a time constant of 120 and 100 ps for [Re(Cl)(CO)3(papy)2] and [Re(papy)(CO)3(bpy)]+, respectively. It is concluded that coordination of papy to the Re center accelerates the ISC, switching the photochemistry from singlet to triplet excited states. Comparison with analogous 4-styrylpyridine complexes (M. Busby, P. Matousek, M. Towrie, A. Vlček Jr., J. Phys. Chem. A 109 (2005) 3000) reveals similarities of the decay mechanism of excited states of Re complexes with ligands containing –NN– and –CC– bonds. Both involve sub-picosecond ISC to triplets, partial rotation around the double bond and slower ISC to the trans or cis ground state. This process is about 200 times faster for the –NN– bonded papy ligand. The intramolecular energy transfer from the 3MLCT-excited ∗Re(CO)3(bpy) chromophore to the intraligand state of the axial ligand occurs for both L = stpy and papy with a comparable rate of a few ps.