Emission from the Charge Transfer State of Xanthene Dye-Sensitized TiO₂ Nanoparticles: A New Approach to Determining Back Electron Transfer Rate and Verifying the Marcus Inverted Regime

Electron injection and back electron transfer dynamics of xanthene dyes adsorbed on TiO2 nanoparticles have been studied by picosecond transient absorption and time-resolved fluorescence spectroscopy. When the xanthene dyes are adsorbed on the TiO2 surface, a good fraction of the dye molecules forms charge transfer (CT) complex with the TiO2 nanoparticle. On excitation of the above system, electron transfer from dye molecule to nanoparticle takes place. Electron injection has been observed by direct detection of electron in the conduction band of nanoparticle and bleach of the dye as detected by picosecond transient absorption spectroscopy. The corresponding dynamics have been determined by monitoring the recovery kinetics of the bleach of the dye in the visible region. Electron injection in the above systems can take place in two different ways:  (1) through the excited state of the dye and (2) through direct injection to the conduction band on excitation of the charge transfer complex. For the charge transfer complex, when the recombination reaction takes place, charge transfer (CT) emission has been observed. Monitoring the CT emission, we have determined the back ET rate. We have also found that the back ET rate for the xanthene dye-sensitized TiO2 CT complex decreases as the relative driving force increases. Assuming a negligible change in electronic coupling, our results provide the evidence for the Marcus inverted region kinetic behavior for an interfacial ET process

Effect of Surface Modification on Back Electron Transfer Dynamics of Dibromo Fluorescein Sensitized TiO₂ Nanoparticles

Electron injection and back electron transfer (BET) dynamics have been carried out for dibromo fluorescein (DBF) sensitized TiO2 nanoparticles capped (modified) with sodium dodecyl benzene sulfonate using transient absorption techniques in picosecond and microsecond time domain. BET dynamics have been compared with bare (unmodified) nanoparticles for the same DBF/TiO2 system. It has been observed that BET reaction is slow on the modified surface compared to a bare surface in earlier time domain (picosecond). This observation has been explained by the fact that on surface modification the energy levels of the semiconductor nanoparticles are pushed up in energy. As a result, the free energy of reaction (−ΔG°) for BET reaction of a dye/SM-TiO2 system increases as compared to the dye/bare TiO2 system. High exoergic BET reaction in dye-sensitized TiO2 nanoparticles surfaces fall in the Marcus inverted regime, so with increasing free energy of reaction, BET rate decreases on the modified surface. However, a reversible trend in BET dynamics has been observed for the above systems in the longer time domain (microsecond). In microsecond time domain BET reaction is faster on the modified surface as compared to on the bare surface. Modification of this surface reduces the density of deep trap states. Recombination dynamics between deep-trapped electron and parent cation is slow due to low coupling strength of BET reaction. As the density of deep-trapped electrons is high in bare particles, BET reaction is slow in longer time domain

Iodine as an efficient catalyst in ionic Diels–Alder reactions of ?,?-unsaturated acetals

A variety of protected and unprotected ,-unsaturated aldehydes react with 1,3-dienes in the presence of I2 to give the corresponding cycloadducts.</p

Electron Injection into the Surface States of ZrO₂ Nanoparticles from Photoexcited Quinizarin and Its Derivatives: Effect of Surface Modification

The effect of surface modification on interfacial electron transfer (IET) dynamics into the surface states of ZrO2 nanoparticles sensitized by quinizarin (Qz) and its derivatives has been carried out using time-resolved emission spectroscopy. The surface of ZrO2 nanoparticles has been modified by sodium dodecyl benzyl sulfonate . We have observed that Qz's can form a strong charge-transfer (CT) complex with both unmodified and surface-modified (SM) ZrO2 nanoparticles. We have confirmed electron injection into the surface states of ZrO2 nanoparticles from the photoexcited Qz molecule in our earlier work (J. Phys. Chem. B 2004, 108, 4775; Langmuir 2004, 20, 7342). In the present investigation, we have observed electron injection from photoexcited Qz derivatives into the surface states of both unmodified and SM ZrO2 nanoparticles and also detected CT emission. Monitoring CT emission, we have determined back electron transfer (BET) dynamics of the dye−nanoparticle systems. We have found that the BET rate for the QZs/ZrO2 systems decreases as the relative driving force increases following Marcus inverted region kinetic behavior for an IET process. BET dynamics was found to be faster on SM ZrO2 nanoparticles as compared to that of the unmodified (bare) one. Our time-resolved emission data indicates that upon surface modification the majority of the deeper trap states of ZrO2 nanoparticles can be removed with the formation of some new shallower trap states in the band gap region

Effect of Molecular Structure on Interfacial Electron Transfer Dynamics of 7-<i>N</i>,<i>N</i>-Dimethyl Coumarin 4-Acetic Acid (DMACA) and 7-Hydroxy Coumarin 4-Acetic Acid (HCA) Sensitized TiO₂ and ZrO₂ Nanoparticles

Ultrafast transient absorption spectroscopy has been employed to understand the effect of molecular structure on interfacial electron transfer (ET) dynamics of 7-N,N-dimethyl amino coumarin 4-acetic acid (DMACA) and 7-hydroxy coumarin 4-acetic acid (HCA) sensitized TiO2 and ZrO2 nanoparticles. Electron injection is confirmed by observing the cation radical of the dye molecules as well as the conduction band electron in the visible and near-IR regions. Electron injection efficiency has been found to be higher for the HCA/TiO2 system as compared to the DMACA/TiO2 system. Both dyes are structurally similar except that HCA has a hydroxyl group at the 7-position while DMACA has a dimethyl amino group at the 7-position. Steady-state and time-resolved fluorescence measurements confirmed that, in highly polar solvent, the excited state of DMACA dye exists both in twisted intramolecular charge transfer (TICT) and intramolecular charge transfer (ICT) states, whereas excited HCA exists only in the ICT state. Because the charge in the case of the TICT state of DMACA is localized away from the nanoparticle surface, electron injection from that state is not efficient. However, ICT states of both DMACA and HCA can inject electrons efficiently as the charge is delocalized. Hence, the quantum yield of electron injection is high in the case of HCA compared to DMACA. We have also observed that photoexcited DMACA whose energy level lies above the conduction band of ZrO2 nanoparticle can inject electrons after exciting with 400 nm laser light. Back electron transfer (BET) rates have been determined by following the decay kinetics of the cation radical and conduction band electron in different systems. The BET rate is found to be slower for HCA/TiO2 compared to DMACA/TiO2, as they fall in the inverted region of Marcus electron transfer theory. The BET is faster in the DMACA/TiO2 system as compared to that of the DMACA/ZrO2 system. However, the residual absorption after 460 ps is less in the case of the DMACA/ZrO2 system

Iodine as an efficient catalyst in ionic Diels–Alder reactions of ?,?-unsaturated acetals

A variety of protected and unprotected ,-unsaturated aldehydes react with 1,3-dienes in the presence of I2 to give the corresponding cycloadducts.</p

Effect of Molecular Structure on Interfacial Electron Transfer Dynamics of 7-<i>N</i>,<i>N</i>-Dimethyl Coumarin 4-Acetic Acid (DMACA) and 7-Hydroxy Coumarin 4-Acetic Acid (HCA) Sensitized TiO₂ and ZrO₂ Nanoparticles

Ultrafast transient absorption spectroscopy has been employed to understand the effect of molecular structure on interfacial electron transfer (ET) dynamics of 7-N,N-dimethyl amino coumarin 4-acetic acid (DMACA) and 7-hydroxy coumarin 4-acetic acid (HCA) sensitized TiO2 and ZrO2 nanoparticles. Electron injection is confirmed by observing the cation radical of the dye molecules as well as the conduction band electron in the visible and near-IR regions. Electron injection efficiency has been found to be higher for the HCA/TiO2 system as compared to the DMACA/TiO2 system. Both dyes are structurally similar except that HCA has a hydroxyl group at the 7-position while DMACA has a dimethyl amino group at the 7-position. Steady-state and time-resolved fluorescence measurements confirmed that, in highly polar solvent, the excited state of DMACA dye exists both in twisted intramolecular charge transfer (TICT) and intramolecular charge transfer (ICT) states, whereas excited HCA exists only in the ICT state. Because the charge in the case of the TICT state of DMACA is localized away from the nanoparticle surface, electron injection from that state is not efficient. However, ICT states of both DMACA and HCA can inject electrons efficiently as the charge is delocalized. Hence, the quantum yield of electron injection is high in the case of HCA compared to DMACA. We have also observed that photoexcited DMACA whose energy level lies above the conduction band of ZrO2 nanoparticle can inject electrons after exciting with 400 nm laser light. Back electron transfer (BET) rates have been determined by following the decay kinetics of the cation radical and conduction band electron in different systems. The BET rate is found to be slower for HCA/TiO2 compared to DMACA/TiO2, as they fall in the inverted region of Marcus electron transfer theory. The BET is faster in the DMACA/TiO2 system as compared to that of the DMACA/ZrO2 system. However, the residual absorption after 460 ps is less in the case of the DMACA/ZrO2 system

Dynamics of Interfacial Electron Transfer from Photoexcited Quinizarin (Qz) into the Conduction Band of TiO₂ and Surface States of ZrO₂ Nanoparticles

Electron injection and back-electron-transfer (BET) dynamics of quinizarin (Qz) adsorbed on TiO2 and ZrO2 nanoparticles has been studied by femtosecond transient absorption spectroscopy in the visible and near-IR region. A good fraction of Qz forms a charge-transfer (CT) complex while being adsorbed on the TiO2 or ZrO2 nanoparticles surface. Following photoexcitation of Qz/TiO2 and Qz/ZrO2 systems, electron injection into the nanoparticles has been confirmed for both the systems by direct detection of electron in the nanoparticle and cation radical of Qz (Qz.+). The dynamics of BET from TiO2 and ZrO2 to the parent cation has been measured by monitoring the decay kinetics of Qz•+ and electron in the nanoparticles and it is found to be multiexponential. As S1 state of Qz lies below the conduction band edge of ZrO2 so electron injection from S1 state into the nanoparticle is not thermodynamically possible. However, the detection of Qz•+ as well as injected electrons in the case of Qz/ZrO2 system confirms that electron injection also takes place in ZrO2. We have attributed this to the injection into surface states of ZrO2 nanoparticles. It has been observed that electron injection takes place in <50 fs and the majority of the injected electrons come back to the parent cation with a time constant of <1 ps for both the systems. We have observed multiphasic recombination dynamics with time constants ranging from ∼600 fs to the pico-, nano-, and microsecond time scale for both Qz/TiO2 and Qz/ZrO2 systems. Our investigation has revealed that electron injection into the surface states of nanoparticles is possible or facilitated when the adsorbed sensitizer molecule forms a strong CT complex with the semiconductor nanoparticles

Physicochemical and Photophysical Studies on Porphyrin-Based Donor−Acceptor Systems: Effect of Redox Potentials on Ultrafast Electron-Transfer Dynamics

We report new polychromophoric complexes, where different porphyrin (P) derivatives are covalently coupled to a redox active Mo center, MoL*(NO)Cl(X) (L* is the face-capping tridentate ligand tris(3,5-dimethylpyrazolyl) hydroborate and X is a phenoxide/pyridyl/amido derivative of porphyrin). The luminescence quantum yields of the bichromophoric systems (1, 2, and 5) were found to be an order of magnitude less than those of their respective porphyrin precursors. Transient absorption measurements revealed the formation of the porphyrin radical cation species (P•+) and photoinduced electron transfer from the porphyrin moiety to the respective Mo center in 1, 2, and 5. Electrochemical studies showed that the reduction potentials of the acceptor Mo centers in a newly synthesized pyridyl derivative (2; E1/2[MoI/0] = ∼ −1.4 V vs Ag/AgCl) and previously reported phenoxy- (1; E1/2[MoII/I] = ∼ −0.3 V vs Ag/AgCl) and amido- (3; E1/2[MoII/I] = ∼ −0.82 V vs Ag/AgCl) derivatives were varied over a wide range. Thus, studies with these complexes permitted us to correlate the probable effect of this potential gradient on the electron-transfer dynamics. Time-resolved absorption studies, following excitation at the Soret band of the porphyrin fragment in complexes 1, 2, and 5, established that forward electron transfer took place biexponentially from both S2 and S1 states of the porphyrin center to the Mo moiety with time constants 150−250 fs and 8−20 ps, respectively. In the case of MoL*(NO)ClX (where X is pyridine derivative 2), the high reduction potential for the MoI/0 couple allowed electron transfer solely from the S2 state of the porphyrin center. Time constants for the charge recombination process for all complexes were found to be 150−300 ps. Further, electrochemical and EPR studies with the trichromophoric complexes (3 and 4) revealed that the orthogonal orientation of the peripheral phenoxy/pyridyl rings negated the possibility of any electronic interaction between two paramagnetic Mo centers in the ground state and thereby the spin exchange, which otherwise was observed for related Mo complexes when two Mo centers are separated by a polyene system with comparable or larger separation distances

Additional file 3: of Genome-wide identification and characterization of InDels and SNPs in Glycine max and Glycine soja for contrasting seed permeability traits

Identification of total SNPs in G. soja with respect to reference genome. (XLSX 19576 kb