Interaction of Zwitterionic Osmolyte Trimethylamine N-oxide (TMAO) with Molecular Hydrophobes: An Interplay of Hydrophobic and Electrostatic Interactions.

Interaction of trimethylamine N-oxide (TMAO) with charged/uncharged moieties of proteins and lipids is an important elementary step toward the multifaceted biofunctions of TMAO. Using minimum area Raman difference spectroscopy (MA-RDS) of aqueous TMAO (1.0 M) in the presence of deuterated molecular hydrophobes (e.g., deuterated tetramethylammonium cation (d-TMA+) and tert-butylalcohol (d-TBA)), we show that TMAO exhibits two distinct motifs of interaction with the cationic (d-TMA+) and uncharged (d-TBA) hydrophobes. Specifically, the trimethylammonium moiety of TMAO undergoes van der Waals attraction with the tert-butyl group of d-TBA, which is governed by their mutual hydrophobic interaction with water. This makes their methyl groups less exposed to water. In contrast, for the cationic hydrophobe (d-TMA+), TMAO interacts electrostatically via its negatively charged-oxygen, which in turn orients the TMAO-methyls away from the hydrophobe (d-TMA+), keeping them exposed to water.

Restructuring of Hydration Shell Water due to Solvent-Shared Ion Pairing (SSIP): A Case Study of Aqueous MgCl2 and LaCl3 Solutions.

Hydration of ions plays a crucial role in interionic interactions and associated processes in aqueous media, but selective probing of the hydration shell water is nontrivial. Here, we introduce Raman difference with simultaneous curve fitting (RD-SCF) analysis to extract the OH-stretch spectrum of hydration shell water, not only for the fully hydrated ions (Mg2+, La3+, and Cl-) but also for the ion pairs. RD-SCF analyses of diluted MgCl2 (0.18 M) and LaCl3 (0.12 M) solutions relative to aqueous NaCl of equivalent Cl- concentrations provide the OH-stretch spectra of water in the hydration shells of fully hydrated Mg2+ and La3+ cations relative to that of Na+. Integrated intensities of the hydration shell spectra of Mg2+ and La3+ ions increase linearly with the salt concentration (up to 2.0 M MgCl2 and 1.3 M LaCl3), which suggests no contact ion pair (CIP) formation in the MgCl2 and LaCl3 solutions. Nevertheless, the band shapes of the cation hydration shell spectra show a growing signature of Cl--associated water with the rising salt concentration, which is a manifestation of the formation of a solvent-shared ion pair (SSIP). The OH-stretch spectrum of the shared/intervening water in the SSIP, retrieved by second-round RD-SCF analysis (2RD-SCF), shows that the average H-bonding of the shared water is weaker than that of the hydration water of the fully hydrated cation (Mg2+ or La3+) but stronger than that of the anion (Cl-). The shared water displays an overall second-order dependence on the concentration of the interacting ions, unveiling 1:1 stoichiometry of the SSIP formed between Mg2+ and Cl- as well as La3+ and Cl-.

Synergistic enhancement in the drug sequestration power and reduction in the cytotoxicity of surfactants.

Surfactants have often been employed for the sequestration of drugs from DNA. However, for an effective sequestration, the concentration of the surfactant needs to be higher than its critical micellar concentration (CMC). Use of such high concentrations of the surfactant may limit its practical usage as a sequestering agent due to its cytotoxicity. In the present study we have shown that sodium dodecyl sulfate (SDS) itself at a concentration less than its CMC failed to sequester a drug from DNA. However, the sequestration power of SDS at sub-CMC concentration could be enhanced to a significant extent when incorporated into Pluronic polymer micelles in the form of supramolecular assemblies. Such a sequestration process was monitored through detailed photophysical properties of a model drug using steady-state and time-resolved fluorescence techniques. It has also been demonstrated that unlike a conventional surfactant, the sequestration of drugs by SDS-polymer supramolecular assemblies can be controlled by their compositions. Two Pluronic polymers with different compositions have been used to understand the effect of polymer composition on the sequestration process. It has been shown that with the increase in the length of the hydrophilic blocks of the polymer, the extent of sequestration decreases due to the decrease in the sequestering force exerted on the intercalated drug. Most importantly, our in vitro cell viability studies show that the toxicity of the SDS surfactant is reduced to a remarkable extent due to its incorporation into the polymer micelles.

Ultrafast Dynamics of Hydrogen Bond Breaking and Making in the Excited State of Fluoren-9-one: Time-Resolved Visible Pump-IR Probe Spectroscopic Study.

The fluoren-9-one (FL) molecule, with a single hydrogen bond-accepting site (C═O group), has been used as a probe for investigation of the dynamics of a hydrogen bond in its lowest excited singlet (S1) state using the subpicosecond time-resolved visible pump-IR probe spectroscopic technique. In 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), a strong hydrogen bond-donating solvent, the formation of an FL-alcohol hydrogen-bonded complex in the ground electronic (S0) state is nearly complete, with a negligible concentration of the FL molecule remaining free in solution. In addition to the presence of a band due to the hydrogen-bonded complex in the transient IR spectrum recorded immediately after photoexcitation of FL in HFIP solution, appearance of the absorption band due to a free C═O stretch provides confirmatory evidence of ultrafast photodissociation of hydrogen bonds in some of the complexes formed in the S0 state. The peak-shift dynamics of the C═O stretch bands reveal two major relaxation pathways, namely, vibrational relaxation in the S1 state of the free FL molecules and the solvent reorganization process in the hydrogen-bonded complex. The latter process follows bimodal exponential dynamics involving hydrogen bond-making and hydrogen bond-reorganization processes. The similar lifetimes of the S1 states of the FL molecules, both free and hydrogen-bonded, suggest establishment of a dynamic equilibrium between these two species in the excited state. However, investigations in two other weaker hydrogen bond-donating solvents, namely, trifluoroethanol (TFE) and perdeuterated methanol (CD3OD), reveal different features of peak-shift dynamics because of the prominence of the vibrational relaxation process over the hydrogen bond-reorganization process during the early time.

Interaction of a Julolidine-Based Neutral Ultrafast Molecular Rotor with Natural DNA: Spectroscopic and Molecular Docking Studies.

Ultrafast molecular rotors (UMRs) are reported to be one of the best fluorescent sensors to study different microenvironments, including biomolecules. In the present work, we have explored the possibility of application of a julolidine-based neutral UMR, 9-(2,2-dicyano vinyl) julolidine (DCVJ), as a DNA sensor and studied its mode of binding with DNA in detail using spectroscopic and molecular docking techniques. Our spectroscopic studies indicate that association of DCVJ with DNA leads to a very large enhancement in its emission intensity. Detailed investigation reveals that, despite being a neutral molecule, binding of DCVJ with DNA is largely modulated in the presence of salt. Such an unusual salt effect has been explained by invoking the ion-dipole interaction between DCVJ and the phosphate backbone of DNA. The ion-dipole interaction has also been established by studying the interaction of DCVJ with nucleosides. Detailed time-resolved studies show that the twisting motion around the vinyl bond in DCVJ gets retarded to a great extent because of its association with DNA molecules. Through competitive binding studies, it has also been established that DCVJ also binds to DNA through intercalation. Finally, quantum chemical calculations and molecular docking studies have been performed to confirm the mode of binding of DCVJ with DNA.

Solvent sensitive intramolecular charge transfer dynamics in the excited states of 4-N,N-dimethylamino-4'-nitrobiphenyl.

Organic molecules substituted with the nitro group show efficient nonlinear optical (NLO) properties, which are a consequence of the strong intramolecular charge transfer (ICT) character of the molecules because of the strong electron withdrawing nature of the nitro group and rapid responsiveness because of highly movable π-electrons. Dynamics of the ICT process in the excited states of a push-pull biphenyl derivative, namely, 4-N,N-dimethylamino-4'-nitrobiphenyl (DNBP), an efficient NLO material, has been investigated using ultrafast transient absorption spectroscopy. The experimental results have been corroborated with DFT and TDDFT calculations. In solvents of large polarity, e.g. acetonitrile, the ultrafast ICT process of DNBP is associated with the barrierless twisting of the N,N-dimethylaniline (DMA) group with respect to the nitrobenzene moiety to populate the twisted ICT (or TICT) state, and the rate of this process is solely governed by the viscosity of the medium. In solvents of moderate polarity, e.g. ethyl acetate, the rate of the twisting process is significantly slowed down and the LE and TICT states remain in equilibrium because of a low energy barrier for interconversion between these two states. By further lowering the polarity of the solvent, e.g. in dioxane, the twisting process is completely retarded. In nonpolar solvents, e.g. cyclohexane, a reverse twisting motion towards the planar geometry (i.e. the PICT process) has been evident in the excited state dynamics. In this solvent, the S1 state undergoes an ultrafast intersystem crossing to the triplet state because of its close proximity with the T2 state.

Effect of Donor-Acceptor Coupling on TICT Dynamics in the Excited States of Two Dimethylamine Substituted Chalcones.

Significant effect of coupling between the electron donor and acceptor groups in intramolecular charge transfer (ICT) dynamics has been demonstrated by comparing the photophysical properties of two isomeric N,N-dimethylaminochalcone derivatives (namely, DMAC-A and DMAC-B). In the case of the DMAC-B molecule, the distance between the donor (N,N-dimethylaniline or DMA) and the acceptor (carbonyl) groups is larger by one ethylene unit as compared to that in the case of DMAC-A. The excited singlet (S1) states of both the isomers have strong ICT character but their photophysical properties are remarkably different. In polar solvents, fluorescence quantum yields (and the lifetimes of the S1 state) of DMAC-A are more than 2 orders of magnitude lower (and shorter) than those of DMAC-B. Remarkable differences in the photophysical properties of these two isomers arise due to occurrence of the ultrafast twisting of the DMA group (or the TICT process) during the course of deactivation of the S1 state of the DMAC-A molecule, but not in the case of DMAC-B. In the later case, because of the presence of a large energy barrier along the twisting coordinate(s), TICT is not a feasible process, and hence, the S1 state of DMAC-B has the planar ICT structure. In the DMAC-A molecule, the strength of coupling between the donor and acceptor groups is relatively stronger because of a shorter distance between these groups. Femtosecond transient absorption spectroscopic measurements and DFT/TDDFT calculations have been adopted to establish the above aspects of the relaxation dynamics of the S1 states of these two isomeric chalcones.

Ultrafast twisting dynamics of thioflavin-T: spectroscopy of the twisted intramolecular charge-transfer state.

Understanding the excited-state properties of thioflavin-T (ThT) has been of immense importance, because of its efficient amyloid-sensing ability related to neurodegenerative disorders. The excited-state dynamics of ThT is studied by using sub-pico- and nanosecond time-resolved transient absorption techniques as well as density functional theory (DFT)/time-dependent DFT calculations. Barrierless twisting around the central C-C bond between two aromatic moieties is the dominant process that contributes to the ultrafast dynamics of the S1 state. The spectroscopic properties of the intramolecular charge-transfer state are characterized for the first time. The energetics of the S0 and S1 states has also been correlated with the experimentally observed spectroscopic parameters and structural dynamics. A longer-lived transient state populated with a very low yield has been characterized as the triplet state.

Probing excited state charge transfer dynamics in a heteroleptic ruthenium complex.

Dynamics of metal to ligand charge transfer in the excited states of ruthenium polypyridyl complexes, which have shown promise as materials for artificial solar energy harvesting, has been of immense interest recently. Mixed ligand complexes are especially important for broader absorption in the visible region. Dynamics of ultrafast vibrational energy relaxation and inter-ligand charge transfer processes in the excited states of a heteroleptic ruthenium complex, [Ru(bpy)2(pap)](ClO4)2 (where bpy is 2,2'-bipyridine and pap is 2-(phenylazo)pyridine) have been investigated using femtosecond to nanosecond time-resolved transient absorption spectroscopic techniques. A good agreement between the TA spectrum of the lowest excited (3)MLCT state of [Ru(bpy)2(pap)](ClO4)2 complex and the anion radical spectrum of the pap ligand, which has been generated using the pulse radiolysis technique, confirmed the charge localization at the pap ligand. While the lifetime of the inter-ligand charge transfer from the bpy to the pap ligand in the (3)MLCT state is about 2.5 ps, vibrational cooling of the pap-localized(3)MLCT state occurs over a much longer time scale with a lifetime of about 35 ps. Ultrafast charge localization dynamics observed here may have important consequences in artificial solar energy harvesting systems, which employ heteroleptic ruthenium complexes.

Picosecond pulse radiolysis study of dynamics of solvation of electron and fluorenone anion in primary alcohols.

We have studied the dynamics of solvation of electron injected directly into primary alcohols as well as that of fluorenone anion using pulse radiolysis technique with the time resolution of about 15 ps. Unlike in the previous reports, we observe nonexponential dynamics of both electron and anion solvation. While the ultrafast component, τ1 (<15 ps) representing the inertial time scale of the dynamics is faster than the time resolution of the spectrometer, the slower component, τ2, has been assigned to the translational motion leading to structural changes of the hydrogen bonding network of the solvent in the inner solvation cell or alcohol cluster. τ2 agrees well with the electron solvation times reported by the earlier authors. τ3 is associated with the restructuring of the hydrogen bond network structure of the solvent in the region outside the solvation cell. Nonexponential solvation dynamics of the fluorenone anion has been described well by a two-component process. The most important observation in this work is that the lifetime of the shorter component, τ1, determined in four alcoholic solvents, is much longer than the electron solvation time in the corresponding solvents determined in this work or anion solvation time reported earlier. The lifetime of this component is nearly comparable with the average dipolar solvation time but shorter than the longitudinal relaxation time of the solvent. In the case of anion, τ1 has been assigned to the restructuring of the first solvation shell by breakage of solvent hydrogen bonds of the fluorenone molecule and formation of hydrogen bonds with the anion. In this case, too, the longer component, τ2, with the lifetime of a few nanoseconds, has been assigned to reorganization of hydrogen bonds in the solvent hydrogen bond network structure.

Dynamics of solvent controlled excited state intramolecular proton transfer coupled charge transfer reactions.

Coupled intramolecular proton and charge transfer reactions play an important role in many biological and chemical reactions. In this article, we report the relaxation dynamics of the excited states of a donor substituted 1,3-diketone (DMADK, ) using steady state and ultrafast transient absorption and fluorescence spectroscopic techniques. Dramatic dependence of the fluorescence quantum yield, nonradiative rate as well as the excited state relaxation pathways on solvent polarity reveals the solvent controlled excited state intramolecular proton transfer (ESIPT) directed intramolecular charge transfer (ICT) dynamics. The molecules in the ground state coexist in two possible cis-enol (Enol-A and Enol-B) forms. Time-dependent density functional theory (TDDFT) calculations reveal solvent polarity controlled thermodynamic stabilization of one of the tautomeric structures in the S1 state, dictating the direction of proton transfer and subsequent structural relaxation. In low and medium polarity solvents, the S1 state of Enol-B (Enol-B*) undergoes ultrafast ESIPT leading to the population of Enol-A*, followed by the ICT process. In polar solvents, the ESIPT process is reversed to populate Enol-B*, which undergoes a twisted intramolecular charge transfer (TICT) process via twisting of the N,N-dimethylaniline group. This work demonstrates that the strength of electronic coupling between the donor and acceptor group determines the structure of the ICT state.

In stationary regime, electron transfer rates in RTIL media are diffusion controlled: experimental evidence from pulse radiolysis study.

We report electron transfer (ET) process from the long-lived radical anions of pyrene and benzophenone to molecular acceptors, e.g., benzophenone and fluorenone, respectively, in two RTIL media, namely, [BMIM][PF6] and [BMIM][BF4], as well as a few other conventional organic solvents using the nanosecond pulse radiolysis technique. Decay of the donor radical anion and concomitant formation of the acceptor radical anion ensure a bimolecular ET process. The rate constants for the bimolecular ET process in both normal organic solvents and RTILs have been found to be nearly equal to diffusion controlled rate calculated for the corresponding solvent. For long-lived anions, having lifetimes longer than a few hundred nanoseconds, quenching occurs mainly in the stationary regime. In this regime, the ET rate is fully controlled by the rate of diffusion of the reactive species in those solvents. To the best of our knowledge, this is the first experimental evidence of the diffusion controlled ET process occurring in the stationary regime in RTIL media.

Photoisomerization dynamics of N-1-methyl-2-(tolylazo) imidazole and the effect of complexation with Cu(II).

Azo-compounds containing an imidazole moiety have the potential to photoregulate biofunctions, such as gene-expression and enzymatic action. Photoinduced isomerization of the azo-backbone is the vital process for such applications, but the photoisomerization dynamics of azo-imidazole compounds has not been well explored. We investigated the photoisomerization dynamics of trans-N-1-methyl-2-(tolylazo) imidazole (trans-MTAI) using femtosecond transient absorption spectroscopy following photoexcitation to the S(2) state. Time evolution of the transient electronic spectra and the global analysis of the temporal profiles reveal a three state relaxation (S(2) → S(1) → S(0)) process in different kinds of solvents. The lifetime of the S(2) state is independent of the viscosity of solvent, whereas that of the S(1) state becomes longer with increasing solvent viscosity. This observation clearly indicates that the large amplitude motion that leads to the trans → cis isomerization occurs only in the S(1) state and relaxation of the S(2) state is not associated with the isomerization process. We have also investigated the excited state dynamics of [Cu(trans-MTAI)(2)]Cl(2) to examine the effect of complexation with the metal ion on the isomerization dynamics of trans-MTAI. It is observed that the photoinduced isomerization of the azo-backbone in trans-MTAI is completely inhibited upon complexation with Cu(II).

Ultrafast dynamics of the excited states of 1-(p-nitrophenyl)-2-(hydroxymethyl)pyrrolidine.

The dynamics of the excited states of 1-(p-nitrophenyl)-2-(hydroxymethyl)pyrrolidine (p-NPP) has been investigated using the subpicosecond transient absorption spectroscopic technique in different kinds of solvents. Following photoexcitation using 400 nm light, conformational relaxation via twisting of the nitro group, internal conversion (IC) and the intersystem crossing (ISC) processes have been established to be the three major relaxation pathways responsible for the ultrafast deactivation of the excited singlet (S(1)) state. Although the nitro-twisting process has been observed in all kinds of solvents, the relative probability of the occurrence of the other two processes has been found to be extremely sensitive to solvent polarity, because of alteration of the relative energies of the S(1) and the triplet (T(n)) states. In the solvents of lower polarity, the ISC is predominant over the IC process, because of near isoenergeticity of the S(1)(ππ*) and T(3)(nπ*) states. On the other hand, in the solvents of very large polarity, the energy of the S(1)(ππ*) state becomes lower than those of both the T(3)(nπ*) and T(2)(nπ*/ππ*) states, but those of the T(1)(ππ*) state and the IC process to the ground electronic (S(0)) state are predominant over the ISC, and hence the triplet yield is nearly negligible. However, in the solvents of medium polarity, the S(1) and T(2) states become isoenergetic and the deactivation of the S(1) state is directed to both the IC and ISC channels. In the solvents of low and medium polarity, following the ISC process, the excited states undergo IC, vibrational relaxation, and solvation in the triplet manifold. On the other hand, following the IC process in the Franck-Condon region of the S(0) state, the vibrationally hot molecules with the twisted nitro group subsequently undergo the reverse nitro-twisting process via dissipation of the excess vibrational energy to the solvent or vibrational cooling.

Ultrafast twisting dynamics in the excited state of auramine.

Relaxation dynamics of the excited state of bis-[4-(dimethylamino)-phenyl] methaniminium chloride (Auramine) has been investigated using subpicosecond time-resolved absorption spectroscopic technique in both aprotic and alcoholic solvents. The locally excited (LE) state, formed following photoexcitation of Auramine using 400 nm light, undergoes intramolecular charge transfer (ICT) process, which is accompanied by the twisting of the dimethylanilino groups. Time evolution of the transient absorption-stimulated emission spectra as well as the wavelength dependence of the temporal dynamics investigated in each kind of solvents suggest that the relaxation process proceeds via the formation of at least two transient states (TS I and TS II), which are geometrical conformers and consecutively formed following the decay of the LE state. Twisting of the dimethylaniline groups are nearly barrierless processes, the rates of which show linear correlation both with the macroscopic or shear viscosities as well as the solvation times of the solvents. Time-dependent and fractional viscosity dependence of the relaxation rates of the LE and the TS I states in aprotic solvents suggest the multidimensionality of the reaction coordinate as well as reveal the viscoelastic property of the solvents. However, in normal alcohols, in addition to these two factors, activation energy of the solvent viscosity may be another important factor for the slower twisting dynamics of Auramine in alcohols. To explain the viscosity dependence of the decay time of the TS II state, which undergoes an efficient internal conversion process to the ground state, the possibility of occurrence of different mechanisms, such as, energy gap law, involvement of intramolecular high frequency modes, as well as the phenyl group twisting motion on a potential energy surface having a photochemical funnel, have been discussed. TDDFT method has been applied to obtain the optimized electronic structures of the transient states but it has been possible to obtain only that for the TS II state.

Ultrafast dynamics of the excited states of curcumin in solution.

Dynamics of the excited singlet (S(1)) state of curcumin has been investigated in a wide varieties of solvents using subpicosecond time-resolved fluorescence and absorption spectroscopic techniques. As a consequence of extra stability of the cis-enol conformer due to the presence of an intramolecular hydrogen bond, it is the major form existing in the ground-state and the excited-state processes described here has been attributed to this form. Steady-state absorption and fluorescence spectra suggest significant perturbation of the intramolecular hydrogen bond and the possibility of formation of intermolecular hydrogen-bonded complex with the hydrogen-bonding solvents. Both the time-resolved techniques used here reveal that solvation is the major process contributing to the relaxation dynamics of the S(1) state. Solvation dynamics in protic solvents is multimodal, and the linear correlation between the longest component of the solvation process and the longitudinal relaxation time of the solvent suggests the specific hydrogen-bonding interaction between the solute and the solvent. However, a good correlation between the experimentally determined average solvation time and that predicted by the dielectric continuum model in all kinds of solvents also suggests that the dielectric relaxation of the solvent is also an important contributor to the solvation process. The lifetime of the S(1) state is very short in nonpolar solvents (∼44 ps in 1,4-dioxane) because of efficient nonradiative deactivation of the S(1) state, which is an important consequence of the ultrafast excited-state intramolecular hydrogen transfer (ESIHT) reaction in the six-membered hydrogen-bonded chelate ring of the cis-enol form. However, it has not been possible to monitor the ESIHT reaction in real time because of the symmetrical structure of the molecule with respect to the hydrogen-bonded chelate ring. In polar solvents, dipole-dipole interaction perturbs the intramolecular hydrogen bond leading to the reduced efficiency of the nonradiative deactivation process. However, stretching vibration in the intermolecular hydrogen bonds formed in the hydrogen-bonding (both donating and accepting) solvents induces another efficient channel for the nonradiative relaxation of the S(1) state of curcumin.

Charge carrier dynamics in thiol capped CdTe quantum dots.

We report the ultrafast charge carrier relaxation dynamics of mercaptopropionic acid capped CdTe quantum dot (QD) using femtosecond transient absorption spectroscopy by exciting the particles with 400 nm laser light and monitoring the transients in the visible to near IR region. Cooling dynamics and population dynamics in different quantized states of the charge carriers were monitored by following the growth kinetics of the bleach at different excitonic positions. The cooling time second and first excitonic states were found to be 150 fs and 500 fs, respectively, which increases non-linearly with its size. Defect states of QD surface play an important role in the cooling dynamics of the charge carriers. Quenching studies have been carried out to find out cooling and trapping dynamics of the individual charge carriers. Electron and hole cooling time were measured to be 700 fs and 150 fs for the first excitonic state using quenchers. Trapping dynamics of electron and hole have been determined by monitoring transient signal at 1000 nm and by using hole and electron quencher, respectively. Electron and hole trapping times have been found to be 700 fs and 1 ps, respectively, in CdTe QD.

Ultrafast dynamics of the excited states of the uranyl ion in solutions.

We have investigated the relaxation dynamics of the higher excited states of the uranyl ion in aqueous and methanolic solutions following photoexcitation to the S(1)((1)Phi(g)) state using 400 nm light. Although the time-resolved spectra are significantly different in these two solvents, the temporal dynamics studied in the entire wavelength region clearly suggest the involvement of three excited state processes in both solvents. The S(1)((1)Phi(g)) state undergoes ultrafast intersystem crossing (tau(ISC) approximately <100 fs) to the higher vibrational levels of the T(2)((3)Delta(g)) state, followed by the intramolecular vibrational relaxation (IVR) process in the later electronic state (tau(IVR) approximately 0.85 and 1 ps in aqueous and methanolic solutions, respectively). Subsequently, the T(2)((3)Delta(g)) state undergoes an internal conversion (IC) process (tau(IC) approximately l.6 and 4.5 ps in aqueous and methanol solutions, respectively) to the long-lived T(1)((3)Phi(g)) state, which is responsible for the luminescent properties of the uranyl ion. In neat methanol, because of stronger interaction between the excited triplet, T(1)((3)Phi(g)), state and the solvent via solvent to uranyl charge transfer, the U(VI) ion undergoes partial reduction to U(V) and the energy level of this state possibly lies lower than that of (UO(2)(2+))*, which is the transient species existing in aqueous solution, and hence increasing the energy gap between the T(2) and T(1) states in methanol solution. These facts possibly explain different spectral characteristics of the transient species produced in methanol and aqueous solutions as well as the longer lifetime of the IC process in methanol solution.

The effect of heavy atoms on photoinduced electron injection from nonthermalized and thermalized donor states of M(II)-polypyridyl (M=Ru/Os) complexes to nanoparticulate TiO(2) surfaces: an ultrafast time-resolved absorption study.

We have synthesized ruthenium(II)- and osmium(II)-polypyridyl complexes ([M(bpy)(2)L](2+), in which M=Os(II) or Ru(II), bpy=2,2'-bipyridyl, and L=4-(2,2'-bipyridinyl-4-yl)benzene-1,2-diol) and studied the interfacial electron-transfer process on a TiO(2) nanoparticle surface using femtosecond transient-absorption spectroscopy. Ruthenium(II)- and osmium(II)-based dyes have a similar molecular structure; nevertheless, we have observed quite different interfacial electron-transfer dynamics (both forward and backward). In the case of the Ru(II)/TiO(2) system, single-exponential electron injection takes place from photoexcited nonthermalized metal-to-ligand charge transfer (MLCT) states. However, in the case of the Os(II)/TiO(2) system, electron injection takes place biexponentially from both nonthermalized and thermalized MLCT states (mainly (3)MLCT states). Larger spin-orbit coupling for the heavier transition-metal osmium, relative to that of ruthenium, accounts for the more efficient population of the (3)MLCT states in the Os(II)-based dye during the electron-injection process that yields biexponential dynamics. Our results tend to suggest that appropriately designed Os(II)-polypyridyl dye can be a better sensitizer molecule relative to its Ru(II) analogue not only due to much broader absorption in the visible region of the solar-emission spectrum, but also on account of slower charge recombination.

Ultrafast relaxation dynamics of the excited states of 1-amino- and 1-(N,N-dimethylamino)-fluoren-9-ones.

The dynamics of the excited states of 1-aminofluoren-9-one (1AF) and 1-(N,N-dimethylamino)-fluoren-9-one (1DMAF) are investigated by using steady-state absorption and fluorescence as well as subpicosecond time-resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen-bonded form in aprotic solvents, the excited-state intramolecular proton-transfer reaction is the only relaxation process observed in the excited singlet (S(1)) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen-bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S(1)(LE), or S(1)(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge-transfer, S(1)(TICT), state. A crossing between the excited-state and ground-state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S(1)(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen-bond-donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen-bonded complex formed between the S(1)(TICT) state and the solvent is possibly avoided and the hydrogen-bonded complex is weakly emissive.