881885-83-2Relevant academic research and scientific papers
Photoinduced intramolecular proton transfer of phenol-containing ligands and their zinc complexes
Goerner, Helmut,Khanra, Somit,Weyhermueller, Thomas,Chaudhuri, Phalguni
, p. 2587 - 2594 (2006)
N,N′-Bis(salicylidene)hydrazine (LI), a bis-2-hydroxybenzene-type ligand H2L, its tert-butyl derivative (LII), 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°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 LI or LII 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.
