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Prashant V Kamat - One of the best experts on this subject based on the ideXlab platform.
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ultrafast photophysical investigation of Cresyl Violet aggregates adsorbed onto nanometer sized particles of sno2 and sio2
Journal of Physical Chemistry B, 1997Co-Authors: Ignacio B Martini, Gregory V Hartland, Prashant V KamatAbstract:The relaxation dynamics of Cresyl Violet H-aggregate dimers adsorbed onto SnO2 or SiO2 colloidal particles has been examined with ca. 200 fs time resolution. These experiments were performed by monitoring both the ground state recovery of the excited dye molecules and the transient absorption signal in the region of the dye radical cation. For Cresyl Violet−SiO2, the ground state recovery is a single exponential with a 2.9 ± 0.2 ps time constant. Transient absorption measurements that monitored the excited electronic state of the dye show a similar 2.5 ± 0.4 ps decay. The observed dynamics for Cresyl Violet−SiO2 is assigned to internal conversion followed by vibrational relaxation of the adsorbed Cresyl Violet dimers. The similarity of the transient absorption and bleach recovery time constants shows that vibrational relaxation is extremely rapid, i.e., internal conversion is the rate-limiting step. For Cresyl Violet−SnO2, the ground state recovery is biexponential with time constants of 2.4 ± 0.4 ps (∼80...
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dye capped semiconductor nanoclusters role of back electron transfer in the photosensitization of sno2 nanocrystallites with Cresyl Violet aggregates
Journal of Physical Chemistry B, 1997Co-Authors: Di Liu, Richard W Fessenden, Gordon L Hug, Prashant V KamatAbstract:Adsorption of a cationic dye, Cresyl Violet, on SnO2 and SiO2 nanoclusters and nanocrystalline thin films results in the formation of H-aggregates. These dyes are photochemically and electrochemically active and extend the photoresponse of large bandgap semiconductors such as SnO2. Photocurrent generation in dye capped nanocrystalline films of SnO2 has been demonstrated with visible light excitation. A photon-to-photocurrent generation efficiency around 1% has been observed at 510 nm. Back electron transfer between the photoinjected electron and the oxidized sensitizer plays an important role in controlling the efficiency of net electron transfer. Transient absorption and microwave absorption measurements of the dye aggregate capped SnO2 films suggest that the back electron transfer is multiexponential and most is completed within a few hundred nanoseconds. The activation energy of the back electron transfer process is very low (∼1.7 kJ/mol).
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picosecond dynamics of Cresyl Violet h aggregates adsorbed on sio2 and sno2 nanocrystallites
Journal of Chemical Physics, 1996Co-Authors: Di Liu, Prashant V KamatAbstract:The H‐aggregates of Cresyl Violet dye which are formed on the negatively charged SiO2 and SnO2 nanocrystallites, exhibit relatively short‐lived excitonic singlet state with a lifetime of about 35 ps. The difference absorption maximum (λmax=470 nm) is blue shifted compared to the corresponding singlet excited state of monomer (λmax=515 nm). Time‐resolved transient absorption measurements show that these dye aggregates are capable of injecting electrons from the triplet excited state into SnO2 nanocrystallites. The rate constant for heterogeneous electron transfer as measured from the formation of cation radical and electron trapping in SnO2 nanocrystallites was 2.0×108 s−1.
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dye capped semiconductor nanoclusters one electron reduction and oxidation of thionine and Cresyl Violet h aggregates electrostatically bound to sno2 colloids
Langmuir, 1996Co-Authors: Di Liu, Prashant V KamatAbstract:SnO2 colloids are capped with the cationic dyes thionine and Cresyl Violet by electrostatic interaction. The close packing of these dyes on the negatively charged SnO2 colloids resulted in the formation of H-aggregates. One-electron reduction and oxidation of these H-aggregates bound to SnO2 colloids has been investigated by pulse radiolysis. The spectral features of semireduced, •OH-adduct, and semioxidized forms of dye monomers and aggregates are compared by recording time-resolved transient absorption spectra. The bimolecular rate constants for the reaction of dye-capped SnO2 colloids with aqueous electrons (5−6 × 109 M-1 s-1) and •OH radicals (3.5 × 108 M-1 s-1) are nearly an order of magnitude smaller than those for the corresponding reactions of dye monomers. The slower diffusion of colloidal particles is likely to influence the diffusion kinetics of reactions involving dye-capped SnO2 colloids.
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photochemistry on surfaces intermolecular energy and electron transfer processes between excited ru bpy 32 and h aggregates of Cresyl Violet on sio2 and sno2 colloids
The Journal of Physical Chemistry, 1995Co-Authors: Di Liu, Gordon L Hug, Prashant V KamatAbstract:Cresyl Violet, a cationic dye (CV{sup +}), forms H-aggregates on the negatively charged SiO{sub 2} and SnO{sub 2} colloids. These aggregates exhibit broad absorbance around 520 nm. By coadsorbing a sensitizer, Ru(bpy){sub 3}{sup 2+}, we are able to characterize the triplet excited state and reduced form of dye-aggregates on the colloidal SiO{sub 2} and SnO{sub 2} suspensions. On SiO{sub 2} surfaces, the excited state quenching of Ru(bpy){sub 3}{sup 2+} by dye-aggregates proceeds via an energy transfer mechanism. Picosecond laser flash photolysis experiments indicate that such a surface-promoted energy transfer is completed within 20 ps. On the other hand dye-aggregates adsorbed onto SnO{sub 2} colloids undergo photosensitized reduction since the excited sensitizer, Ru(bpy){sub 3} {sup 2+}, is efficiently quenched by the semiconductor support. The role of support material in promoting energy and electron transfer processes is described. 87 refs., 11 figs.
Luchuan Shu - One of the best experts on this subject based on the ideXlab platform.
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mechanism of nonphotochemical hole burning Cresyl Violet in polyvinyl alcohol films
Journal of The Optical Society of America B-optical Physics, 1992Co-Authors: Luchuan Shu, Gerald J. SmallAbstract:Polarized nonphotochemical hole-burned spectra of Cresyl Violet in polyvinyl alcohol films are presented for various burn temperatures (TB). For TB = 15 K, but not for TB = 2.2 K, a significant rotation of Cresyl Violet is indicated. The broad, tailing antihole is observed to depend on TB. The quantum efficiency distributions for the zero-phonon hole and pseudo-phonon sideband are quite similar, proving that the phonons created by excitation of the phonon sideband are not important for hole burning. The quantum efficiency is independent of TB over the range studied, 1.6–15 K. Over this range essentially 100% of the zero-phonon lines can be burned. The results show that the standard two-level-system (extrinsic) model for nonphotochemical hole burning is inadequate. The results are discussed in terms of an outside-in hierarchy of configurational relaxation events model [ Chem. Phys.141, 447 ( 1990)] as well as other models.
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laser induced hole filling Cresyl Violet in polyvinyl alcohol films
Journal of The Optical Society of America B-optical Physics, 1992Co-Authors: Luchuan Shu, Gerald J. SmallAbstract:Laser-induced hole filling results are reported for the case in which the primary burn frequency, ωB, and the secondary (filling) frequency, ωS, lie in the origin absorption band of Cresyl Violet. Both two-laser and one-laser (with a Fourier-transform spectrometer) experiments were performed. The one-laser experiments permit monitoring of the entire hole spectrum, which consists of the zero-phonon hole at ωB, its companion phonon sideband holes, and the blue-shifted antihole. From the dependence of the filling efficiency on ωB − ωS it is concluded that the dominant mechanism involves electronic excitation of the antihole sites by means of their broad phonon sideband. It is shown that the antihole sites retain at least a partial memory of their preburn configurations.
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dispersive kinetics of nonphotochemical hole burning and spontaneous hole filling Cresyl Violet in polyvinyl films
Journal of The Optical Society of America B-optical Physics, 1992Co-Authors: Luchuan Shu, G J SmallAbstract:The dispersive kinetics of nonphotochemical burning and spontaneous filling of the zero-phonon hole of Cresyl Violet in polyvinyl alcohol at 1.6 K are analyzed in terms of the standard external two-level system (TLSezt) model for probe–glass systems and a distribution function for the tunnel frequency derived from a normal distribution function for the tunnel parameter λ. Average values for the relaxation rates for burning and filling are determined. It is shown that the dominant mechanism for filling is not global spectral diffusion but rather antihole reversion. A high degree of positive correlation between the rates of burning and filling associated with the TLSext is found. A new methodology that permits a more physically reasonable interpretation of spontaneous hole-filling kinetics is described. It is based on the hypothesis that only a fraction of burned sites, on reversion to the ground state, yield sites with resonance frequencies that lie within the hole profile.
Gerald J. Small - One of the best experts on this subject based on the ideXlab platform.
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mechanism of nonphotochemical hole burning Cresyl Violet in polyvinyl alcohol films
Journal of The Optical Society of America B-optical Physics, 1992Co-Authors: Luchuan Shu, Gerald J. SmallAbstract:Polarized nonphotochemical hole-burned spectra of Cresyl Violet in polyvinyl alcohol films are presented for various burn temperatures (TB). For TB = 15 K, but not for TB = 2.2 K, a significant rotation of Cresyl Violet is indicated. The broad, tailing antihole is observed to depend on TB. The quantum efficiency distributions for the zero-phonon hole and pseudo-phonon sideband are quite similar, proving that the phonons created by excitation of the phonon sideband are not important for hole burning. The quantum efficiency is independent of TB over the range studied, 1.6–15 K. Over this range essentially 100% of the zero-phonon lines can be burned. The results show that the standard two-level-system (extrinsic) model for nonphotochemical hole burning is inadequate. The results are discussed in terms of an outside-in hierarchy of configurational relaxation events model [ Chem. Phys.141, 447 ( 1990)] as well as other models.
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laser induced hole filling Cresyl Violet in polyvinyl alcohol films
Journal of The Optical Society of America B-optical Physics, 1992Co-Authors: Luchuan Shu, Gerald J. SmallAbstract:Laser-induced hole filling results are reported for the case in which the primary burn frequency, ωB, and the secondary (filling) frequency, ωS, lie in the origin absorption band of Cresyl Violet. Both two-laser and one-laser (with a Fourier-transform spectrometer) experiments were performed. The one-laser experiments permit monitoring of the entire hole spectrum, which consists of the zero-phonon hole at ωB, its companion phonon sideband holes, and the blue-shifted antihole. From the dependence of the filling efficiency on ωB − ωS it is concluded that the dominant mechanism involves electronic excitation of the antihole sites by means of their broad phonon sideband. It is shown that the antihole sites retain at least a partial memory of their preburn configurations.
Di Liu - One of the best experts on this subject based on the ideXlab platform.
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dye capped semiconductor nanoclusters role of back electron transfer in the photosensitization of sno2 nanocrystallites with Cresyl Violet aggregates
Journal of Physical Chemistry B, 1997Co-Authors: Di Liu, Richard W Fessenden, Gordon L Hug, Prashant V KamatAbstract:Adsorption of a cationic dye, Cresyl Violet, on SnO2 and SiO2 nanoclusters and nanocrystalline thin films results in the formation of H-aggregates. These dyes are photochemically and electrochemically active and extend the photoresponse of large bandgap semiconductors such as SnO2. Photocurrent generation in dye capped nanocrystalline films of SnO2 has been demonstrated with visible light excitation. A photon-to-photocurrent generation efficiency around 1% has been observed at 510 nm. Back electron transfer between the photoinjected electron and the oxidized sensitizer plays an important role in controlling the efficiency of net electron transfer. Transient absorption and microwave absorption measurements of the dye aggregate capped SnO2 films suggest that the back electron transfer is multiexponential and most is completed within a few hundred nanoseconds. The activation energy of the back electron transfer process is very low (∼1.7 kJ/mol).
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picosecond dynamics of Cresyl Violet h aggregates adsorbed on sio2 and sno2 nanocrystallites
Journal of Chemical Physics, 1996Co-Authors: Di Liu, Prashant V KamatAbstract:The H‐aggregates of Cresyl Violet dye which are formed on the negatively charged SiO2 and SnO2 nanocrystallites, exhibit relatively short‐lived excitonic singlet state with a lifetime of about 35 ps. The difference absorption maximum (λmax=470 nm) is blue shifted compared to the corresponding singlet excited state of monomer (λmax=515 nm). Time‐resolved transient absorption measurements show that these dye aggregates are capable of injecting electrons from the triplet excited state into SnO2 nanocrystallites. The rate constant for heterogeneous electron transfer as measured from the formation of cation radical and electron trapping in SnO2 nanocrystallites was 2.0×108 s−1.
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dye capped semiconductor nanoclusters one electron reduction and oxidation of thionine and Cresyl Violet h aggregates electrostatically bound to sno2 colloids
Langmuir, 1996Co-Authors: Di Liu, Prashant V KamatAbstract:SnO2 colloids are capped with the cationic dyes thionine and Cresyl Violet by electrostatic interaction. The close packing of these dyes on the negatively charged SnO2 colloids resulted in the formation of H-aggregates. One-electron reduction and oxidation of these H-aggregates bound to SnO2 colloids has been investigated by pulse radiolysis. The spectral features of semireduced, •OH-adduct, and semioxidized forms of dye monomers and aggregates are compared by recording time-resolved transient absorption spectra. The bimolecular rate constants for the reaction of dye-capped SnO2 colloids with aqueous electrons (5−6 × 109 M-1 s-1) and •OH radicals (3.5 × 108 M-1 s-1) are nearly an order of magnitude smaller than those for the corresponding reactions of dye monomers. The slower diffusion of colloidal particles is likely to influence the diffusion kinetics of reactions involving dye-capped SnO2 colloids.
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photochemistry on surfaces intermolecular energy and electron transfer processes between excited ru bpy 32 and h aggregates of Cresyl Violet on sio2 and sno2 colloids
The Journal of Physical Chemistry, 1995Co-Authors: Di Liu, Gordon L Hug, Prashant V KamatAbstract:Cresyl Violet, a cationic dye (CV{sup +}), forms H-aggregates on the negatively charged SiO{sub 2} and SnO{sub 2} colloids. These aggregates exhibit broad absorbance around 520 nm. By coadsorbing a sensitizer, Ru(bpy){sub 3}{sup 2+}, we are able to characterize the triplet excited state and reduced form of dye-aggregates on the colloidal SiO{sub 2} and SnO{sub 2} suspensions. On SiO{sub 2} surfaces, the excited state quenching of Ru(bpy){sub 3}{sup 2+} by dye-aggregates proceeds via an energy transfer mechanism. Picosecond laser flash photolysis experiments indicate that such a surface-promoted energy transfer is completed within 20 ps. On the other hand dye-aggregates adsorbed onto SnO{sub 2} colloids undergo photosensitized reduction since the excited sensitizer, Ru(bpy){sub 3} {sup 2+}, is efficiently quenched by the semiconductor support. The role of support material in promoting energy and electron transfer processes is described. 87 refs., 11 figs.
G J Small - One of the best experts on this subject based on the ideXlab platform.
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dispersive kinetics of nonphotochemical hole burning and spontaneous hole filling Cresyl Violet in polyvinyl films
Journal of The Optical Society of America B-optical Physics, 1992Co-Authors: Luchuan Shu, G J SmallAbstract:The dispersive kinetics of nonphotochemical burning and spontaneous filling of the zero-phonon hole of Cresyl Violet in polyvinyl alcohol at 1.6 K are analyzed in terms of the standard external two-level system (TLSezt) model for probe–glass systems and a distribution function for the tunnel frequency derived from a normal distribution function for the tunnel parameter λ. Average values for the relaxation rates for burning and filling are determined. It is shown that the dominant mechanism for filling is not global spectral diffusion but rather antihole reversion. A high degree of positive correlation between the rates of burning and filling associated with the TLSext is found. A new methodology that permits a more physically reasonable interpretation of spontaneous hole-filling kinetics is described. It is based on the hypothesis that only a fraction of burned sites, on reversion to the ground state, yield sites with resonance frequencies that lie within the hole profile.