Studies of the Photophysical Properties and Kinetics in Photo-Induced Isomerization, Excited-State Intramolecular Proton Transfer and the Heavy-Metal Enhanced Phosphorescence
Date Issued
2008
Date
2008
Author(s)
Lin, Chun-Wei
Abstract
Part I. The strategic design of a Salen ligand possessing strong excited-state charge transfer properties leads to a previously unrecognized feature in the photo-induced isomerization. The synthesis of 2,3-bis(4-(di-p-tolylamino)-2-hydroxybenzylideneamino) maleonitrile (1) rendered Z and E conformers in the absence and presence of room light, respectively. We conclude that the 1Z → 1E isomerization in the excited state is triggered by the rotation of the maleonitrile C(2)-C(3) (1Z) bond, which becomes single-bond like due to the photoinduced charge transfer reaction. art II. The dual function of the excited-state intramolecular proton transfer (ESIPT) in the newly synthesized 3-hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one has been recognized. One of the two ESIPT routes typically happens between the hydroxyl group and the carbonyl group in a five-membered ring configuration. The other occurs between the hydroxyl group and the pyridinic group in a six-membered ring configuration. The switchable ESIPT implies the potential applications of the pH fluorescent indicator and the anion chemosensor. The dual function in the 3-hydroxyflavone derivative is believed to further spark large interests for the utilization of the ESIPT in a more practical way. art III. Cyclometalated osmium complexes with the formulas [Os(ppy)2(CO)2] (1a,b), [Os(dfppy)2(CO)2] (2a,b), and [Os(btfppy)2(CO)2] (3a,b) have been synthesized, for which the chelating chromophores ppyH, dfppyH, and btfppyH denote 2-phenylpyridine, 2-(2,4-difluorophenyl)pyridine, and 2-(2,4-bis(trifluoromethyl)phenyl)pyridine, respectively. UV–vis and emission spectra were measured, revealing the lowest excited state for all complexes as a nominally ligand-centered 3ππ* state mixed with certain MLCT character. Introduction of the electron-withdrawing substituents on the cyclometalated chelates or replacement of one CO ligand with phosphine at the metal center increased the MLCT contribution in the first excited state, giving a broad and featureless emission with greatly enhanced quantum yields.art IV. A series of Pt(II) complexes Pt(fpbpy)Cl (1), Pt(fpbpy)(OAc) (2), Pt(fpbpy)(NHCOMe) (3), Pt(fpbpy)(NHCOEt) (4) and [Pt(fpbpy)(NCMe)](BF4) (5) with deprotonated 6-(5-trifluoromethyl-pyrazol-3-yl)-2,2’-bipyridine terdentate ligand were prepared, among which 1 was converted to complexes 2 ~ 5 via simple ligand substitution. In contrast to 3, which exhibits concentration-independent red phosphorescence at 604 nm, complex 1 shows strong concentration-dependent emission, giving normal (560 nm) and excimeric-like dual emission (~660 nm), manifesting its strong Pt(II)-Pt(II) interaction in the exited state. In sharp contrast, complex 3 exhibits ligand centered ππ* phosphorescence at 505 nm in solid state versus red shifted emission at 604 nm recorded in CH2Cl2 solution.
Subjects
Salen ligand
photo-induced isomerization
excited-stateintramolecular proton transfer
3-hydroxyflavone
phosphorescence
Osmium(II)complex
platinum(II) complex
Type
thesis
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