Decreasing Excitation Wavelength Research Articles

Photoinduced Ligand Redistribution Chemistry of Quadruply Bonded Mo2Cl2(6-mhp)2(PR3)2 Complexes

The quadruply bonded metal-metal complexes cis-Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) (R(3) = Et(3), Me(3), Me(2)Ph, MePh(2); 6-mhp = 2-hydroxy-6-methylpyridinato) photoreact when their solutions are irradiated with visible and near-UV light. The primary photoprocess leads to the ligand redistribution products Mo(2)Cl(3)(6-mhp)(PR(3))(3) and Mo(2)Cl(6-mhp)(3)(PR(3)). In THF at room temperature, these photoproducts are stable and over time they back-react completely to the starting material. Photolysis of cis-Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) in DMF results in the same products; however, Mo(2)Cl(3)(6-mhp)(PR(3))(3) rapidly decomposes, leaving Mo(2)Cl(6-mhp)(3)(PR(3)) as the only isolable photoproduct. Conversely, when the reaction is carried out in benzene, Mo(2)Cl(6-mhp)(3)(PR(3)) undergoes a slow secondary photoreaction and Mo(2)Cl(3)(6-mhp)(PR(3))(3) is the photoproduct that is isolated. At a given wavelength, the photolysis quantum yield (Phi(p)) increases along the solvent series C(6)H(6) < THF < DMF (Phi(p)(405) = 0.00042, 0.00064, and 0.00097, respectively, for cis-Mo(2)Cl(2)(6-mhp)(2)(PMe(2)Ph)(2)). For a given solvent, Phi(p) increases with decreasing excitation wavelength (Phi(p)(546) = 0.00012, Phi(p)(436) = 0.00035, Phi(p)(405) = 0.00042, Phi(p)(366) = 0.0022, and Phi(p)(313) = 0.0079 in C(6)H(6)). This wavelength dependence of the photoreaction quantum yield in conjunction with the excitation spectrum establishes that the photoreaction does not originate from the lowest energy deltadelta excited state, which possesses a long lifetime and an appreciable emission quantum yield in C(6)H(6), CH(2)Cl(2), THF, and DMF. The photochemistry is instead derived from higher energy excited states with the maximum photoreactivity observed for excitation wavelengths coinciding with absorption features previously assigned to ligand-to-metal charge transfer transitions.

Photoinduced Ligand Redistribution Chemistry of Quadruply Bonded Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) Complexes.

The quadruply bonded metal-metal complexes cis-Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) (R(3) = Et(3), Me(3), Me(2)Ph, MePh(2); 6-mhp = 2-hydroxy-6-methylpyridinato) photoreact when their solutions are irradiated with visible and near-UV light. The primary photoprocess leads to the ligand redistribution products Mo(2)Cl(3)(6-mhp)(PR(3))(3) and Mo(2)Cl(6-mhp)(3)(PR(3)). In THF at room temperature, these photoproducts are stable and over time they back-react completely to the starting material. Photolysis of cis-Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) in DMF results in the same products; however, Mo(2)Cl(3)(6-mhp)(PR(3))(3) rapidly decomposes, leaving Mo(2)Cl(6-mhp)(3)(PR(3)) as the only isolable photoproduct. Conversely, when the reaction is carried out in benzene, Mo(2)Cl(6-mhp)(3)(PR(3)) undergoes a slow secondary photoreaction and Mo(2)Cl(3)(6-mhp)(PR(3))(3) is the photoproduct that is isolated. At a given wavelength, the photolysis quantum yield (Phi(p)) increases along the solvent series C(6)H(6) < THF < DMF (Phi(p)(405) = 0.00042, 0.00064, and 0.00097, respectively, for cis-Mo(2)Cl(2)(6-mhp)(2)(PMe(2)Ph)(2)). For a given solvent, Phi(p) increases with decreasing excitation wavelength (Phi(p)(546) = 0.00012, Phi(p)(436) = 0.00035, Phi(p)(405) = 0.00042, Phi(p)(366) = 0.0022, and Phi(p)(313) = 0.0079 in C(6)H(6)). This wavelength dependence of the photoreaction quantum yield in conjunction with the excitation spectrum establishes that the photoreaction does not originate from the lowest energy deltadelta excited state, which possesses a long lifetime and an appreciable emission quantum yield in C(6)H(6), CH(2)Cl(2), THF, and DMF. The photochemistry is instead derived from higher energy excited states with the maximum photoreactivity observed for excitation wavelengths coinciding with absorption features previously assigned to ligand-to-metal charge transfer transitions.

Photophysical triplet state processes of 4H-1-benzopyrane-4-thione in a perfluoroalkane. Part 3.—Excitation-wavelength dependence of luminescence quantum yields

The phosphorescence excitation spectrum of 4H-1-benzopyrane-4-thione (BPT) in perfluoro-1,3-dimethylcyclohexane coincides with the absorption spectrum in the range of the transitions S0→ T1 and S0→ S1. Hence the quantum yield ϕP of the phosphorescence from BPT is independent of the primary excited state, T1 or S1, in contrast to a reported lower value of ϕP for excitation to S1. The ratio ϕPF2/ϕP of the quantum yields ϕPF2 of the prompt S2→ S0 fluorescence and ϕP of the phosphorescence decreases with decreasing excitation wavelength.

Polarization of porous silicon luminescence

The polarization characteristics of light emitting porous Si films is investigated. The degree of polarization increases to well over 20% when the excitation wavelength is at the blue edge of the emission band and decreases with decreasing excitation wavelength. The steady-state value of the degree of polarization is established in less than 1 μs. The experimental results bear qualitative agreement with a quantum-wire model.

Dependence of the excitation wavelength on the Raman-active phonons of YBa 2Cu 3O 7: Search for Landau damping in single-domain crystals

In single-domain crystals of YBa 2Cu 3O 7, we examine the dependence of phonon linewidth on wavevector by varying the wavelength of the exciting laser. In three crystals, we find the linewidths of the Raman-active phonons at 120 and 150 cm −1 to be nearly invariant for excitation wavelengths between 647 and 413 nm. That is, we find no broadening of the phonon linewidths with increasing phonon wavevector (decreasing excitation wavelength), and thus no evidence of Landau damping in our crystals. In addition, the correlation between the intensity of the chain-related Raman feature at ≈232 cm −1 and the temperature dependence of the B 1g-like phonon at 340 cm −1 has been investigated. In all three crystals, we find a net sharpening of the 340 cm −1 phonon below T c and essentially no intensity for the ≈232 cm −1 mode under resonance conditions.

Extrinsic Carrier-photogeneration in Poly(N-vinylcarbazole)

Abstract The carrier-photogeneration efficiency of holes in poly(N-vinylcarbazole) (PVCz) films was investigated by means of the time-of-flight method as a function of applied electric field, temperature, and excitation band. The extrinsic effects on the efficiency were examined; these included photooxidation of film surface and doping with a weak electron-acceptor, dimethyl terephthalate (DMTP). The experimental results were analyzed according to the Onsager theory of geminate recombination of a bound electron-hole pair with an interionic separation of ro. The primary yield of the electron-hole pair, φo, increased from 0.02 to 0.059 with decreasing excitation wavelength. By photooxidation, it increased up to 0.09 for all the excitation bands. The ro was independent of the excitation band, and was identical between fresh and photooxidized films. In DMTP(1.8 mol%)-doped films, both φo and ro were independent of the excitation band, although their values were different from those of undoped films. These results indicate that the extrinsic carrier-photogeneration via nonrelaxed exciplexes formed between singlet excitons and acceptor-like impurities or photooxidation products is predominant in PVCz films for all the excitation bands.

Temperature dependence of a process competing with S2-S1 internal conversion in indole and phenol in aqueous solutions

The drop in phi/sub f/ with decreasing excitation wavelength, previously attributed to ionization from the S/sub 2/ state, increases substantially with increasing temperature for both indole and phenol in aqueous solution. Addition of 2-propanol to the solvent reduces the drop in phi/sub f/ but not its activation energy which on the other hand depends significantly on the solute. phi/sub eaq/-measured upon excitation of the S/sub 2/ state of indole in H/sub 2/O increases from 0.09 at 25/sup 0/C to 0.17 at 65/sup 0/C. The results are interpreted in terms of an intermediate charge transfer to solvent state (CTTS) populated from S/sub 2/. In the 2-propanol-water solvent the formation of this CTTS state appears to be governed primarily by interaction of the excited solute with water molecules. (auth)