Picosecond Laser Flash Photolysis Research Articles

Photochemistry of 2-Nitroarenes: 2-Nitrophenyl-α-trifluoromethyl Carbinols as Synthons for Fluoroorganics.

Facile synthesis of a new series of 2,2'-bis(trifluoroacetyl) azoxybenzene derivatives and trifluoromethylated benzo[c]isoxazoline systems, along with trifluoroacetyl nitrosobenzene derivatives was achieved by solvent controlled photolysis of appropriate 2-nitrobenzyl alcohols. Corresponding photoactive 2-nitrobenzyl chromophore plays a distinct role in this photosynthetic process, while, quite unprecedented, pertinent fluoromethyl substitution leads to high value fluoromethylated products, whose direct access is not feasible by common synthetic protocols. The significance of fluorine and fluoroalkyl substitution and its prominent biological effects makes this new photochemical approach an important discovery in synthetic methodology. Plausible mechanistic pathways involved in the formation of the products during steady-state photolysis are further established by picosecond laser flash photolysis experiments.

ChemInform Abstract: Photochemistry of Sensitizing Dyes. Spectroscopic and Redox Properties of Cresyl Violet

The photosensitizing properties of cresyl violet have been investigated by characterizing singlet and triplet excited states and the reduced and oxidized forms of the dye by picosecond and nanosecond laser flash photolysis. The dye readily forms a charge-transfer complex with amines with complexation constants in the range of 49-3,350 M{sup {minus}1} in methanol. The dye in its singlet and triplet excited states has lifetimes of 2.46 ns and 68 {mu}s, respectively, in acetonitrile. The excited triplet is quenched by ground-state dye molecules with a bimolecular rate constant of 1.45 {times} 10{sup 9} M{sup {minus}1} s{sup {minus}1}. The semioxidized radical of the dye has been generated by oxidation of CV{sup +} with pulse radiolytically generated azide radicals (k = 1.6 {times} 10{sup 10} M{sup {minus}1} s{sup {minus}1}) in aqueous medium. The semireduced dye has been generated by quenching of triplet dye with triphenylamine (k = 1.4 {times} 10{sup 9} M{sup {minus}1} s{sup {minus}1}) in methanol. Photoelectrochemical reduction of cresyl violet in colloidal TiO{sub 2} suspension has also been carried out, with a quantum yield of 0.013.

Deactivation of the first excited singlet state of thiophenols

On the bases of picosecond and nanosecond laser flash photolysis with detection by emission and absorption spectroscopy, a quantitative description is given of all deactivation channels of the first excited singlet state of thiophenols ArSH(S(1)) such as fluorescence, intersystem crossing (ISC), chemical dissociation into radicals, and radiation-less internal conversion (IC). For this purpose, the photolysis of thiophenol and its methyl-, methoxy-, and chloro-substituted derivatives was studied in solvents of increasing polarity: 1-chlorobutane, ethanol, and acetonitrile. The fluorescence lifetime of the thiophenols was found to range from some hundreds of picoseconds up to a few nanoseconds, correlating with fluorescence quantum yields between 0.001-0.040, at room temperature. Depending on the substitution pattern of the aromatic ring, the quantum yield of the S-H bond dissociation was found to be between 0.3-0.5, irrespective of the solvent polarity. In laser photolysis, no triplet formation of the investigated compounds could be observed neither by the direct way nor by subsequent sensitization with beta-carotene. As a difference to the total, the radiation-less internal conversion (Phi(IC)>or= 0.5) was found to be the dominating process.

Quantitative Description of the Deactivation Channels of the First Excited Singlet State of 2- and 4-Thiosalicylic Acids

On the bases of picosecond and nanosecond laser flash photolysis with detection by emission and absorption spectroscopy, a quantitative description is given of all of the deactivation channels of the first excited singlet state of 2- and 4-thiosalicyclic acids (TS) such as fluorescence, intersystem crossing (ISC), chemical dissociation into radicals, and radiationless internal conversion (IC). For this purpose, the investigated compounds were studied in solvents of increasing polarity: 1-chlorobutane, acetonitrile, ethanol, and water. As an exception for thiols, and in contrast to recent findings on thiophenol and its methyl- and methoxy-substituted derivatives, the photoinduced first excited triplet state of the thiosalicyclic acids was directly observed by its T1-T(n) absorption spectra and characterized by sensitization experiments. According to the direct determination of the quantum yields of all deactivation channels of the first excited singlet state of 2- and 4-thiosalicylic acid isomers, it was found that the compounds studied have small fluorescence quantum yields (phi(F) = 0.003-0.009) and higher ones for triplet formation (phi(T) = 0.10-0.35) and S-H photodissociation (phi(D) = 0.05-0.15). As a difference to the total, the radiationless internal conversion (phi(IC) = 0.6-0.8) was found to be the dominating process.

Correction to: A study of 2-azido-3,5-dichlorobiphenyl by nano- and picosecond laser flash photolysis and computational methods

Correction to: A study of 2-azido-3,5-dichlorobiphenyl by nano- and picosecond laser flash photolysis and computational methods

A study of 2-azido-3,5-dichlorobiphenyl by nano- and picosecond laser flash photolysis and computational methods

The dynamics of singlet 3,5-dichloro-2-biphenylnitrene and the products of its rearrangement were monitored by pico- and nanosecond laser flash photolysis and the results were consistent with the predictions of DFT and ab initio molecular orbital calculations.

Photodissociation and photoionization mechanisms of 2,2’- and 4,4’-biphenyldiols: a laser flash photolysis study

Nanosecond and picosecond laser flash photolysis (LFP) of 2,2’- and 4,4’-biphenyldiols has been investigated in aqueous acidic and alkaline solutions to understand the details of the photodissociation (PD) and photoionization (PI) mechanisms of the two diols. For 2,2’-biphenyldiol, the photodissociation and photoionization following the excitation using UV light (248 or 266 nm) is seen to be a bi-photonic process, whereas that of the 4,4’-biphenyldiol is seen to follow a monophotonic mechanism. A detailed analysis of the nanosecond and picosecond LFP results indicate that the PD/PI of 2,2’-biphenyldiol involves the participation of the triplet (T1) state of the molecule. For 4,4’-biphenyldiol, however, the PD/PI occurs from the excited singlet (S1) state. It is inferred that the difference in the PD/PI behavior of 2,2’- and 4,4’-biphenyldiols arises due to the presence and absence of intramolecular hydrogen bonding in the two respective molecules. Qualitative potential energy diagrams have been presented to rationalize how the presence and absence of intramolecular hydrogen bonding causes a difference in the stabilization of the electronic states in the two diols and that ultimately determines the possibility of the bi-photonic mono-photonic PD/PI mechanisms for 2,2’- and 4,4’-biphenyldiols, respectively.

Investigation on the properties of the transient absorption of cyclometallated Pt(II) complexes using picosecond and nanosecond laser-flash photolysis

The transient species of the [ppbpy]Pt(II)R (ppbpy = 4-pyrene-6-phenyl-2,2′-bipyridine, Ph—C≡C (PyPPt), R=Cl (PyClPt)) complexes were investigated in solution by employing nanosecond and picosecond pump-probe spectroscopy. The interligand triplet state is found to be main intermediate. The growth of the triplet state is a fast process (< 10 ps), and the lifetime of transient decay is about 7μs, which is matched with the lifetime of luminescence.

Photophysical properties of coumarin-120: Unusual behavior in nonpolar solvents

Photophysical properties of coumarin-120 (C120; 7-amino-4-methyl-1,2-benzopyrone) dye have been investigated in different solvents using steady-state and time-resolved fluorescence and picosecond laser flash photolysis (LFP) and nanosecond pulse radiolysis (PR) techniques. C120 shows unusual photophysical properties in nonpolar solvents compared to those in other solvents of moderate to higher polarities. Where the Stokes shifts (Δν̄=ν̄abs−ν̄fl), fluorescence quantum yields (Φf), and fluorescence lifetimes (τf) show more or less linear correlation with the solvent polarity function Δf={(ε−1)/(2ε+1)−(n2−1)/(2n2+1)}, all these parameters are unusually lower in nonpolar solvents. Unlike in other solvents, both Φf and τf in nonpolar solvents are also strongly temperature dependent. It is indicated that the excited singlet (S1) state of C120 undergoes a fast activation-controlled nonradiative deexcitation in nonpolar solvents, which is absent in all other solvents. LFP and PR studies indicate that the intersystem crossing process is negligible for the present dye in all the solvents studied. Photophysical behavior of C120 in nonpolar solvent has been rationalized assuming that in these solvents the dye exists in a nonpolar structure, with its 7-NH2 group in a pyramidal configuration. In this structure, since the 7-NH2 group is bonded to the 1,2-benzopyrone moiety by a single bond, the former group can undergo a fast flip-flop motion, which in effect causes the fast nonradiative deexcitation of the dye excited state. In moderate to higher polarity solvents, it is indicated that the dye exists in an intramolecular charge-transfer structure, where the bond between 7-NH2 group and the 1,2-benzopyrone moiety attains substantial double bond character. In this structure, the flip-flop motion of the 7-NH2 group is highly restricted and thus there is no fast nonradiative deexcitation process for the excited dye.

Clusters in Polymer−Surfactant AOT Microemulsions Probed by Excited State Quenching Kinetics

Excited-state quenching kinetics were applied to sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) reversed micelles with poly(oxy)ethylene (POE) of molecular weight 35 000 Da where clusters were previously detected [Laia,C. A. T.; Brown ,W.; Almgren, M.; Costa, S. M. B. Langmuir 2000, 16, 465]. The data obtained in AOT reversed micelles, with a fixed value of the molar ratio of water to surfactant (w 0 = [H 2 O]/[AOT]) w 0 = 20, were compared in the absence and presence of the polymer. Two reaction time scales were investigated with different probes: 1,3,6,8-pyrenetetrasulfonic acid (PTSA) in the nanosecond range and Cr(bpy) 3 3+ in the microsecond range, using, respectively, time-resolved fluorescence with picosecond resolution and laser flash photolysis techniques. The experimental decays were analyzed assuming two components, one due to polymer-free micelles described by a Poisson distribution model kinetics and another associated with the polymer-induced micelle clusters, analyzed according to a model based on the concept of random walks in regular compact lattices. A step frequency, k w = 10 7 s - 1 , was extracted from the nanosecond results for the quencher's random walk within the cluster. By contrast, the triplet absorption quenching studies did not distinguish between free micelles and clusters and afforded an aggregation number for the former in the range 260 ′ 40, in good agreement with literature data. Decay simulations confirm the validity of the data treatment used in the nanosecond range, but the short time behavior predicted in the microsecond range was not observed within the equipment time resolution.

Photoexcitation and Electron Transfer Properties of Rod- and Coil-Type Oligo(thienylene−ethynylene)s

Photoexcitation and electron-transfer properties of two series of oligo(thienylene−ethynylene)s, in which thiophene rings were connected with ethynylene groups at 2,5 or 2,3 positions (nαTE or nβTE; n denotes the number of the repeating unit), have been studied. From MO calculations and steady-state absorption spectra, expanded π-electron systems were expected for rod-type nαTE in the ground states, while limited π-electron systems were expected for coil-type nβTE. On the other hand, because nβTE shows a substantial red shift of the fluorescence band similar to that of nαTE with increasing n value, a conformational change expanding π-conjugation of nβTE was suggested in the excited state. From the picosecond laser flash photolysis, the time scale for the conformational change was evaluated to be ca. 30 ps. The triplet state properties of nαTE and nβTE were estimated by means of the nanosecond laser flash photolysis. Furthermore, electron donor abilities of the present oligomers were investigated by studying the photoinduced electron-transfer processes with fullerenes, C60 and C70. It was revealed that the present oligomers donate an electron to the triplet excited C60 or C70 generating the radical cations and anions of oligomers and fullerene, respectively. The electron-transfer rate constants were as small as 0.07−0.0008 of the diffusion-controlled limit, indicating the longer range electron-transfer processes due to larger size of the oligomers and fullerenes. On the other hand, back-electron-transfer processes proceeded at the diffusion-limiting rate.

Magnetic field effects on the hydrogen abstraction reaction of triplet 4-methoxybenzophenone with thiophenol as studied by a picosecond laser flash photolysis technique

The hydrogen abstraction reaction of triplet 4-methoxybenzophenone with thiophenol at 265 K has been studied with a newly developed picosecond laser flash photolysis apparatus under magnetic fields of 0–1.7 T. The decay rate constant of the radical pair generated was found to increase from 3.42 × 109 s−1 to 4.15 × 109 s−1 with increasing the magnetic field from 0 to 1.7 T. The observed magnetic field effects can be explained by the Δg mechanism. Using the simple kinetics model with the Δg mechanism, the rate constant of the escape process from the pair (k esc) and two rate constants for the T-S spin conversion process (k T-S) at 0 and 1.7 T were found to be 1.97 × 109 s−1, 1.45 × 109 s−1, and 2.12 × 109 s−1, respectively. From the magnetic field dependence on k T-S, the difference in the g values of the 4-methoxybenzophenone ketyl and phenylthiyl radicals was estimated to be 0.0087.

Metal–metal and metal–semiconductor composite nanoclusters

Abstract Colloidal metal–metal and metal–semiconductor nanocomposites have been synthesized by the chemical reduction of [AuCl4]− on the surface of preformed Silver and TiO2 colloids and Ag+ on the surface of SnO2 and SiO2 colloids. Photoinduced bleaching of surface plasmon band has been monitored using a picosecond laser flash photolysis apparatus. In the case of Ag/Au nanoparticles, the magnitude of the surface plasmon bleach of the silver core can be modulated by controlling the capping layer of gold without altering the long term bleaching recovery kinetics. In the case of TiO2/Au nanoclusters, the ratio of [TiO2]/[Au] directly influences the particle size and the stability of these core-shell composite nanoparticles. Aggregation effects were observed for colloids with a TiO2:Au ratio of less than 1:1. Laser pulse excitation (532 nm) of these aggregated colloids resulted in the fusion and formation of large nanocomposite clusters. The gold capped TiO2 nanoparticles were found to improve the efficiency of interfacial charge transfer process.

Photoinduced electron transfer from aliphatic amines to coumarin dyes

Electron transfer (ET) interactions of a series of 7-aminocoumarin dyes with aliphatic amine donors have been investigated using steady-state (SS) and time-resolved (TR) fluorescence quenching, picosecond laser flash photolysis (LFP) and pulse radiolysis (PR) techniques. For different coumarin–amine pairs, the estimated quenching constants (kq) from SS and TR fluorescence measurements are found to be similar within the experimental error. That ET from amine donors to excited (S1) coumarin dyes takes place has been established from the LFP and the PR results. For different coumarin–amine pairs, the kq values are seen to correlate well with the free energy changes (ΔG0) for the ET reactions following Marcus’ outer-sphere ET theory. The total reorganization energy (λ) estimated from this correlation is seen to be just similar to the solvent reorganization energy (λs). The leveled-off kq value under diffusion-controlled condition (kqDC) appears to be much lower (∼2.5 times) for the present systems compared to the corresponding value obtained for the ET reactions in coumarin–aromatic amine systems. The large difference in the kqDC values with aliphatic and aromatic amines as the electron donors has been rationalized on the basis of the shapes of the highest occupied molecular orbitals (HOMO) of the amine donors. For aliphatic amines, since their HOMOs are largely localized on the amino group, a large orientation factor is involved in the encounter complexes for the ET reaction to take place. With π like HOMOs, such orientational restriction is just nominal with the aromatic amines as the electron donors. Present ET results under diffusive conditions have also been discussed with a comparison to the ET rates observed under nondiffusive conditions, where the amines are directly used as the solvent donors.

Interparticle electron transfer between size-quantized CdS and TiO2 semiconductor nanoclustersDedicated to Professor Frank Wilkinson on the occasion of his retirement.

Photoinduced electron transfer in a size-quantized CdS/TiO2 composite system has been investigated using emission and transient absorption spectroscopy. Quantum-sized CdS and TiO2 particles were synthesized in reverse micelles using di-octyl sulfosuccinate (Aerosol-OT, AOT) as the surfactant stabilizer. The particle sizes of CdS and TiO2 were controlled by varying water-to-surfactant molar ratio, wo = [H2O]/[AOT], with values of 1, 2.5, 5 and 10. The blue-shift in the absorption onset confirmed size-quantization of these semiconductor particles. Electron transfer from photoexcited CdS to TiO2 was found to depend on the particle size of TiO2, where charge transfer was observed only when TiO2 particles were sufficiently large (>12 A). Interactions with smaller size TiO2 particles (⩽10 A) with CdS instead led to enhancements in emission with an increase in quantum yield from 2.3% to 8.8%. Picosecond laser flash photolysis experiments have been carried out to elucidate the interparticle electron transfer processes in the CdS/TiO2 reverse micellar system.

Photophysical properties of novel water soluble fullerene derivatives

The spectroscopic and photophysical properties for a series of bis-adduct derivatives of C 60 have been studied using a combination of time-resolved and steady state techniques, including picosecond single photon counting and laser flash photolysis. The electronic absorption and fluorescence spectra are red shifted with respect to those of C 60. As with the parent fullerene, the main deactivation channel of the excited singlet state is intersystem crossing with the yields of singlet oxygen ranging from 0.63 to 0.97. Our results demonstrate that the photophysical properties depend on the addition pattern of the addends.

Excited Singlet (S1) State Interactions of 6,11-Dihydroxy-5,12-naphthacenequinone with Aromatic Hydrocarbons

Interaction in the excited singlet state (S1) of 6,11-dihydroxy-5,12-naphthacenequinone (DHNQ) with aromatic hydrocarbons (AH) has been investigated using steady-state (SS) and time-resolved (TR) fluorescence quenching measurements. In both nonpolar (cyclohexane; CH) and polar (acetonitrile; ACN) solvents, the fluorescence quenching of DHNQ by AHs is accompanied with the appearance of exciplex emissions. The emission maxima of the exciplexes correlate linearly with both ionization potentials (IP) and oxidation potentials {E(AH/AH.+)} of the quenchers (AH), indicating the charge transfer (CT) type of interaction between the S1 state of DHNQ (acceptor) and the ground state of the AHs (donor). The kinetic details of the exciplex formation have been evaluated by analyzing the SS and TR fluorescence quenching results at different temperatures following a suitable mechanistic scheme. Picosecond laser flash photolysis (LFP) studies on the DHNQ−AH systems show a major transient absorption band in the 530 to 630 nm region along with a weak long-wavelength absorption tail. The transient lifetimes for the 530−630 nm absorption band are very similar to the exciplex lifetimes estimated from the fluorescence quenching results. At the long wavelength absorption tail, the transient lifetime could not be estimated due to very weak absorption. It is inferred that the 530−630 nm transient absorption band is mostly due to the S1→ Sn transition. The long wavelength absorption tail has been attributed to the anion radical of DHNQ, drawing an analogy with the anion radical absorption spectrum of 1,4-dihydroxy-9,10-anthraquinone (quinizarin; QZ), a lower analogue of DHNQ. The picosecond LFP results largely correlate with the results obtained from the fluorescence quenching studies.

Unusual Photophysical Properties of Coumarin-151

The photophysical properties of 7-amino-4-trifluoromethyl-1,2-benzopyrone (coumarin-151; C151) in nonpolar (NP) solvents are drastically different than those in other solvents of moderate to higher polarities. Thus, the absorption and fluorescence maxima are largely blue-shifted and the Stokes' shifts unusually low in NP solvents compared to those in the other solvents. The fluorescence quantum yields (Φf) in NP solvents are also exceptionally lower. While in all other solvents the fluorescence decays of C151 are single-exponential, in NP solvents the decays show non-single-exponential behavior. The average fluorescence lifetimes (τf) in NP solvents are also substantially shorter. Low Φf and τf values in NP solvents indicate very fast nonradiative deactivation for the S1 state of C151. Investigating the triplet characteristics of C151 by using picosecond laser flash photolysis and pulse radiolysis techniques, it has been established that the fast nonradiative deactivation in NP solvents is not due to an e...

Dye-Capped Gold Nanoclusters: Photoinduced Morphological Changes in Gold/Rhodamine 6G Nanoassemblies

Au nanoparticles (particle diameter ∼2 nm) prepared by thiocyanate reduction method are too small to exhibit the characteristic surface plasmon band. Addition of Rhodamine 6G (Rh 6G) to the colloidal gold solution brings about significant changes in the absorption spectrum. Two distinct peaks appear at 507 and 537 nm. Close packing of the cationic dye molecules on the gold surface induces intermolecular and intercluster interactions. Furthermore, the adsorption of the cationic dye on the gold surface results in surface charge neutralization causing the Au/dye assembly to aggregate. When the Au/Rh-6G solution was subjected to 532 nm laser pulse irradiation for long time intervals, we observed distinct changes in the absorption spectrum due to morphological changes. A growth in the particle size (5−20 nm) is observed as a result of melting and fusion of gold nanoparticles. The photoinduced morphological changes have been elucidated using picosecond laser flash photolysis. The multiphoton process leading to fusion of particles occurs with a rate constant of 7 × 108 s-1. The indirect role of Rh-6G in assisting the fusion of gold nanoparticles is discussed.

Effect of solvent polarity on the aggregation of C 60

C 60 forms aggregates in solution when the dielectric constant of the solvents exceeds some critical value viz. ⩾13. Further, for aggregation the C 60 concentration has to exceed some critical value. The aggregation process is reversible and the aggregates exist in equilibrium with monomers. Picosecond laser flash photolysis experiments indicate that the excited (S 1) aggregates undergo very fast relaxation without contributing towards the triplet formation.