Design, synthesis and studies of novel photochromic zinc(II) thiolate complexes
Abstract of thesis entitled
DESIGN, SYNTHESIS AND STUDIES OF NOVEL PHOTOCHROMIC ZINC(II) THIOLATE COMPLEXES
Ngan Tung Wan
for the degree of Master of Philosophy
at The University of Hong Kong
in August 2004
A series of diarylethene-containing diimine ligands was designed, synthesized and incorporated into the zinc(II) dithiolate system. The zinc(II) diimine dithiolate system was chosen in view of its well-known intense ligand-to-ligand charge transfer (LLCT) luminescence in the visible region. All the ligands and complexes were characterized by 1H NMR spectroscopy, positive FAB- or EI-mass spectrometry and gave satisfactory elemental analyses. The X-ray crystal structure of two of the zinc(II) dithiolate complexes were also determined. The novel phenanthroline-based dithienylethene zinc(II) complexes 1a-1c and 2a-2c displayed low-energy absorption shoulders at ca. 382 nm, tentatively assigned as ligand-to-ligand charge transfer [LLCT, p^SR? - **(L)] transitions. Another series of
zinc(II) diimine complexes, 3a-3c, containing dithienylene-based dipyrido[3,2-a:2?3?c]phenazine ligand showed a broad low-energy absorption band at ca. 420 nm, assigned as a mixture of ligand-to-ligand charge transfer [LLCT, p^SR? -^(dppzL1)] and intraligand [IL, u(dppzL1) - ^(dppzL1) or n - ^(dppzL1)] transitions.
Upon UV excitation at ca. 313 nm, zinc(II) complexes 1a-1c underwent photocyclization reaction, leading to the formation of a new absorption band in the UV region and two moderately intense absorptions in the visible region. The significant bathochromic shift of the absorption maxima corresponding to their open forms was mainly due to the formation of the closed forms, resulting in the increase in the extended p-conjugation across the condensed thiophene rings. In contrast to UV-induced photocyclization, excitation into the absorption bands of the closed form in the visible region resulted in the photochromic backward reaction. The absorption bands of the closed form decreased in intensity, indicating the regeneration of the open form as a result of the photochromic backward reaction. Moreover, the quantum yields for both the forward and backward reactions were determined. The photophysical and electrochemical properties of both the open and closed forms were also studied. Interesting switching of the luminescence and electrochemical properties of the complexes 1a-1c and 2a-2c induced by the photochromic reactions was demonstrated.
The UV-visible absorption spectra of the free ligand (dppzL1) as well as complexes 3a-3c did not show any spectral change upon photo-irradiation at any wavelength. This indicated that the corresponding ligand and complexes are photochemically inactive, which could be explained by the inefficient p-delocalization between the C11 and C12 in the dppz moiety, in which the concerted p-bond rearrangement between C11?12 and the thiophene units could not readily occur with light excitation. The luminescence and electrochemical properties of the free ligand (dppzL1) and its zinc(II) dithiolate complexes 3a-3c were also described. Upon excitation of the ligand and complexes at ? > 350 nm in degassed benzene solution, intense emissions at ca. 473 and 525 nm, respectively, were observed, which were assigned to originate from the respective 3IL and metal-perturbed 3IL states of dppzL1 and complexes 3a-3c.
School:The University of Hong Kong
School Location:China - Hong Kong SAR
Source Type:Master's Thesis
Keywords:zinc compounds photochromic materials ligands
Date of Publication:01/01/2005