- Ultrafast excited state dynamics controlling photochemical isomerization of N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium coordinated to a {Re i(CO)3(2,2′-bipyridine)} chromophore
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Two multifunctional photoactive complexes [Re(Cl)(CO)3- (MeDpe+)2]2+ and [Re(MeDpe+)(CO) 3(bpy)]2+ (MeDpe+ = N-methyl-4-[trans-2-(4- pyridyl)ethenyl]pyridinium, bpy = 2,2′-bipyridine) were synthesized, characterized, and their redox and photonic properties were investigated by cyclic voltammetry; ultraviolet-visible-infrared (UV/Vis/IR) spectroelectrochemistry, stationary UV/Vis and resonance Raman spectroscopy; photolysis; picosecond time-resolved absorption spectroscopy in the visible and infrared regions; and time-resolved resonance Raman spectroscopy. The first reduction step of either complex occurs at about - 1.1 V versus Fc/Fc + and is localized at MeDpe+. Reduction alone does not induce a trans→cis isomerization of MeDpe+. [Re(Cl)(CO) 3(MeDpe+)2]2+ is photostable, while [Re(MeDpe+)(CO)3(bpy)]2+ and free MeDpe + isomerize under near-UV irradiation. The lowest excited state of [Re(Cl)(CO)3(MeDpe+)2]2+ has been identified as the Re(Cl)(CO)3 → MeDpe+ 3MLCT (MLCT = metal-to-ligand charge transfer), decaying directly to the ground state with lifetimes of ≈ 42 (73%) and ≈ 430 ps (27%). Optical excitation of [Re-(MeDpe+)(CO)3(bpy)]2+ leads to population of Re(CO)3→MeDpe+ and Re(CO) 3→bpy 1MLCT states, from which a MeDpe+ localized intraligand 3ππ* excited state (3IL) is populated with lifetimes of ≈ 0.6 and ≈ 10 ps, respectively. The 3IL state undergoes a ≈ 21 ps internal rotation, which eventually produces the cis isomer on a much longer timescale. The different excitedstate behavior of the two complexes and the absence of thermodynamically favorable interligand electron transfer in excited [Re(MeDpe+)(CO) 3(bpy)]2+ reflect the fine energetic balance between excited states of different orbital origin, which can be tuned by subtle structural variations. The complex [Re(MeDpe+)(CO) 3(bpy)]2+ emerges as a prototypical, multifunctional species with complementary redox and photonic behavior.
- Busby, Michael,Hartl, Frantisek,Matousek, Pavel,Towrie, Mike,Vlcek Jr., Antonin
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- EVIDENCE OF THE SINGLET RADICAL PAIR PRECURSOR IN THE PHOTOLYTIC GENERATION OF 1,4-DIMETHYLPYRIDINYL RADICAL FROM ITS DIMER.
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Time-resolved ESR spectra of the 1,4-dimethylpyridinyl radical were observed after the photolysis of its dimer with a N2 laser.Since the transient ESR spectra exhibited the A/E polarization, singlet radical pair is proposed as the precursor of the present radical on the basis of simple sign rule.
- Akiyama, Kimio,Tero-Kubota, Shozo,Ikenoue, Tsuneo,Ikegami, Yusaku
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