Photoinduced intramolecular proton transfer of phenol-containing ligands and their zinc complexes.
N,N'-Bis(salicylidene)hydrazine (L(I)), a bis-2-hydroxybenzene-type ligand H2L, its tert-butyl derivative (L(II)), and the corresponding Zn2+ complexes of the type Zn2(LH)2L (Zn-I and Zn-II) were synthesized. The molecular structure of Zn-II was determined by X-ray crystallography at -170 degrees C. The photoreactions of the four compounds in solution were studied by time-resolved UV-vis spectroscopy using nanosecond laser pulses. A weak but strongly Stokes shifted fluorescence signal of the ligands L(I) or L(II) is suggested to be due to excited-state intramolecular proton transfer (ESIPT) from the phenolic hydroxy group to the nitrogen of the methine bond in analogy to the fast enol --> keto tautomerization of other 2-hydroxybenzenes. A transient with the maximum at 480 nm, bleaching at 370 nm, and a lifetime of 0.01-0.3 ms is attributed to the trans-keto tautomer, formed via internal conversion. The decay occurs via trans --> cis isomerization and proton back-transfer to the enol form. Quenching by water indicates a proton-catalyzed reaction. To account for similar fluorescence and transient properties in the cases of the Zn2+ complexes, a photoinduced tautomerism at one of the two free phenolic hydroxy groups is proposed. The rapid ESIPT followed by a relatively slow relaxation process is reversible.
Max-Planck-Institut für Bioanorganische Chemie, D-45413 Mülheim an der Ruhr, Germany. goerner@mpi-muelheim.mpg.de
Helmut
H
Görner
Sumit
S
Khanra
Thomas
T
Weyhermüller
Phalguni
P
Chaudhuri
D002384:Catalysis; D005453:Fluorescence; D008024:Ligands; D008956:Models, Chemical; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry; D009942:Organometallic Compounds / Q000138:chemical synthesis*; D019800:Phenol / Q000737:chemistry*; D010777:Photochemistry; D011522:Protons; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet; D014867:Water / Q000737:chemistry; D015032:Zinc / Q000737:chemistry*
D008024:Ligands; D009942:Organometallic Compounds; D011522:Protons; D014867:Water; D019800:Phenol; D015032:Zinc; D009584:Nitrogen