Yalin Zhou

Photoinduced interaction between fluorescein ester derivatives and CdS colloid.

The photoinduced interaction of fluorescein ester derivatives, fluoresceins butyl ester (FL4) and fluoresceins anthraquinone-methyl ester (FL-AQ), and colloidal CdS was examined by absorption, fluorescence spectroscopy, and photoinduced ESR spectroscopy. It is found that FL4 and FL-AQ molecules are adsorbed on the surfaces of CdS colloid by an electrostatic interaction forming the surface complex of the type CdS-FL4 or CdS-FL-AQ. The apparent association constant (K(app)) and the degree of association (alpha) of CdS-FL4 obtained from absorption spectra are 2.25 x 10(4) M(-1) and 0.78, respectively. The values of K(app) and alpha of CdS-FL4 as determined from fluorescence spectra are 1.54 x 10(4) M(-1) and 0.82, respectively, which matches well with that determined from the absorption spectra changes. And the values of K(app) and alpha of CdS-FL-AQ obtained from absorption spectra are 4.18 x 10(4) M(-1) and 0.83, respectively. These data indicate that there is a strong interaction between the dye and the CdS particle surface. But there was no evidence for interfacial electron transfer from FL4 or FL-AQ to colloidal CdS by photoinduced ESR experiments. The fluorescence quenching is due to the formation of a nonfluorescent complex. The related phenomena are discussed in this paper.

Photophysical properties of amphiphilic porphyrins in different media

Abstract Photophysical properties (absorption spectra, fluorescence spectra, lifetimes and quantum yields) have been studied for several amphiphilic porphyrins bearing hydrophilic pyridinium and hydrophobic long-chain alkyl groups in different media. These amphiphilic porphyrins are characterized by reduced fluorescence quantum yields, aggregating appreciably in nonpolar solvents and biexponential fluorescence decay. The observed biexponential fluorescence decay of these porphyrin derivatives can be interpreted in terms of mixing of the S1 and a CT state in which an electron is transferred from the porphyrin core to the electron-deficient pyridinium group.

Comparative study of photophysical properties of isomeric tetrapyridyl- and tetra-(N-hexadecylpyridiniumyl) porphyrins

Photophysical properties (absorption spectra, fluorescence spectra, lifetimes and quantum yields) were studied for the three isomers of tetrapyridylporphyrins (TPyPs) in CHCl3 and tetra-(N-hexadecylpyridiniumyl) porphyrins (TC16PyPs) in CHCl3, CH3OH and Triton X-100 micelle solution. While the ground-state and excited-state properties are very similar for the three isomers of neutral TPyP, significant differences exist among the isomers of amphiphilic TC16PyP. Amphiphilic porphyrins bearing hydrophilic pyridinium and hydrophobic long-chain alkyl groups are characterized by reduced fluorescence quantum yields, biexponential fluorescence decay, and appreciable aggregation in nonpolar solvents. The observed biexponential fluorescence decay of amphiphilic porphyrins can be interpreted in terms of mixing of the S1 and a close-lying CT state in which an electron is transferred from the porphyrin core to the electron-deficient pyridinium group.

A study of photoinduced electron transfer and redox properties of hypericin

Abstract Hypericin (HYP), is a derivative of polyhydroxy substituted perylenequinones. In acetonitrile, the fluorescence of HYP can be quenched by electron donor: N,N -diethylaniline (DEA) and acceptors: methyl viologen (MV), anthraquinone (AN) (less than 1.2 × 10 −3 mol l −1 ), and the quenching rate constants are K q (DEA) = 3.52 × 10 10 l mol −1 s −1 K q (MV) = 7.16 × 10 11 l mol −1 s −1 , K q (AN) = 1.05 × 10 11 l mol −1 s −1 respectively, indicating that hypericin can act either as an electron acceptor like quinones or as an electron donor like phenones. Hypericin in ground state maybe forms charge transfer complex with the higher concentrated anthraquinone which has a plane configuration. The stoichiometry is 1:2.6 (HYP:AN) determined by Jobs photometric titration plots. The methyl viologen cation radical formed in the photoinduced interaction between hypericin and methyl viologen was also detected by the absorption spectrum.

Nanosecond time-resolved studies of long-lived photoinduced charge separation in the dyad fluorescein–anthraquinone–methyl ester adsorbed on TiO2 colloids

A dyad composed of fluorescein and 2-methyl-anthraquinone (FL-AQ) was synthesized and its photophysical properties were examined by absorption, fluorescence spectroscopy, and fluorescence lifetime. The charge-separated state formed by photoinduced intramolecular electron transfer was detected by nanosecond transient absorption spectroscopy for the first time. When FL is excited in solution, the photoinduced electron transfer from FL to AQ proceeds efficiently. The rate constant and the efficiency of intramolecular electron transfer are 3.95 x 10(9) s(-1) and 95%, respectively. Its charge-separated state lifetime is too short to detect by transient absorption spectroscopy. Adding nanometer colloidal TiO(2) to an FL-AQ ethanol solution prolongs the lifetime of the charge-separated state, so its transient absorption signal is recorded significantly. The lifetimes of FL(+). at 480 nm and AQ(-). at 560 nm in the FL-AQ/TiO(2) colloidal system are 11.1 and 8.93 mivros, respectively.

Structure and photoelectric properties of tetrakis(4-N-hexadecylpyridiniumyl)porphyrin/anthraquinone LB films

Using a 1∶1 tetrakis(4-N-hexadecylpyridiniumyl)porphyrin/anthraquinone (TC16PyP(4)/AnQ) mixture as film-forming material, the structure and properties of TC16PyP(4)/AnQ mixed monolayers or LB films were studied by π–A isotherms, UV–VIS absorption spectra, and low-angle X-ray diffraction (LAXD). The experimental results indicate that the TC16PyP(4)/AnQ mixture has good film-forming properties at the air/water interface. The TC16PyP(4)/AnQ mixed monolayer can be transferred to glass, quartz or SnO2 optically transparent electrode (OTE) substrates. Most AnQ molecules are incorporated into the cavities between the long alkyl chains of TC16PyP(4) molecules in the mixed monolayer or in LB films. The TC16PyP(4)/AnQ mixed LB films can be regarded as a two-dimensional host–guest system and show good stability and periodicity. The photovoltaic behaviour of an electrochemical cell containing a TC16PyP(4)/AnQ LB film, deposited on an SnO2 OTE, was also investigated. The photocurrent and photovoltage are apparently lower than those for the photocell of pure TC16PyP(4) LB films due to the incorporation of AnQ.

Photoelectric properties of amphiphilic porphyrin Langmuir–Blodgett film

Abstract The photovoltaic behavior of an electrochemical cell of TC 16 PyP(4) monolayer, deposited on SnO 2 optically transparent electrodes (SnO 2 OTE) by the Langmuir–Blodgett (LB) technique was investigated. The photocell was able to generate photocurrent, which increase in the presence of electron donors or acceptors in the KCl electrolyte solution, indicating that the photovoltaic effect of TC 16 PyP(4) perhaps originates from the photo-induced charge transfer between TC 16 PyP(4) molecules and SnO 2 semi-conductor. Moreover, the orientation of TC 16 PyP(4) in monolayer or LB films was also studied by π –A isotherm and polarized absorption spectrum. The result showed that the porphyrin macroring was oriented with an angle of about 25° against the aqueous surface as well as the substrate plane.