Thu-Hoa Tran-Thi

Primary ultrafast events preceding the photoinduced proton transfer from pyranine to water

Abstract Femtosecond fluorescence and absorption spectroscopies are used to probe the early events of the photoinduced proton transfer (PT) from pyranine to water. The process is found to involve two ultrafast steps (300 fs and 2.5 ps) which precede the relatively slow (87 ps) PT step. From the comparative study of the properties of the excited acid and its conjugate anion in various aqueous and alcoholic media, these ultrafast steps are identified as the solvation dynamics of the locally excited (LE) state of the acid and its subsequent relaxation to an intermediate electronic state, whose nature is discussed.

Near infrared absorption spectra of lanthanide bis-phthalocyanines

Abstract Near infrared absorption spectra (700–1800 nm) of various lanthanide bis-phthalocyanines (Pc 2 Lu, Pc 2 Yb, Pc 2 Tm, Pc 2 Dy) show two relatively intense bands (ϵ max ≈10 4 M −1 cm −1 ). The higher energy band is assigned to the 1e g →a lu transition of the radical macrocycle (Pc − ) while the lower energy one may be related to an intramolecular charge transfer from one macrocyclic ligand (Pc 2− ) to the other (Pc − ). The corresponding spectra for the reduced and oxidized species (Pc 2 Ln − , Pc 2 Ln + ) are in agreement with these assignments.

Time-resolved fluorescence spectroscopy of high-lying electronic states of Zn-tetraphenylporphyrin

Depopulation of the S2 excited electronic state of the Zn-tetraphenylporphyrin (ZnTPP) was monitored by measuring the decay of S2→S0 and the rise of S1→S0 fluorescence using the up-conversion fluorescence technique with a time-resolution of 120 fs. The lifetime of the S2 electronic state, measured for ZnTPP in ethanol (τS2=2.35 ps) correlates with the risetime of S1→S0 fluorescence. This result demonstrates the depopulation of S2 to S1 via (vibrational) states with lifetimes much shorter than that of S2. The rise time of S2 fluorescence (τv=60–90 fs) was attributed to vibrational relaxation (in S2). Fluorescence anisotropy decay of the S2 state was also studied by measuring the parallel and perpendicular fluorescence components. The high initial anisotropy of r⩾0.7 is interpreted as due to the existence of a degenerate excited electronic state S2 and the corresponding fast decay time τ1=0.2 ps to the electronic dephasing of the degenerate level pair. The long component of the anisotropy decay (τ2≫10 ps) i...

Intermolecular photochemical proton transfer in solution: new insights and perspectives

Recent theoretical results on the title topic, informed by femtosecond and steady state spectroscopic studies, are reviewed. It is argued that currently employed conceptions of excited state intermolecular proton transfer of the hydroxyarene acid–base type in solution need to be extended or replaced. Among the key features involved are aspects associated with the dynamics—such as the quantum character of the proton nuclear motion and the nature of the reaction coordinate—and the electronic structure description of the reaction process. One example of the latter—the source of the greatly enhanced acidity in the excited state compared to that of the ground state—is given special attention.

Laser-induced intramolecular charge transfer in a lutetium bis-phthalocyanine thin film

Abstract Laser excitation of lutetium bis-phthalocyanine thin film in both the Q band (λ max =670 nm) and the intramolecular charge-transfer band (λ max = 1423 nm) leads to the same transient species, which has been characterized by its optical absorption spectrum and lifetime. The transient species results either from a direct charge-transfer reaction from the di-anion Pc 2− subunit to the radical ion Pc − moiety, or via electronically excited states of Pc 2 Lu. A cooperative molecular orientation is invoked in the ground-state energy recovery process of the transient species.

Chemical sensors of monocyclic aromatic hydrocarbons based on sol–gel materials: kinetics of trapping of the pollutants and sensitivity of the sensor

We report the study of the trapping process of benzene and toluene in porous, hybrid organic–inorganic materials prepared via the sol–gel process. In particular, the kinetics of diffusion of the pollutants within the matrices are studied as functions of the composition, polarity and thickness of the matrix. The concentrations of the pollutants in the air are directly measured via their absorbance in the UV, using three different protocols of exposure of the sensor to the pollutants. A threshold of detection of 60 ppb is achieved when the sensor is a monolith of hybrid TMOS/MeTMOS of stoichiometry 9/1.

Subpicosecond excitation of strongly coupled porphyrin–phthalocyanine mixed dimers

Electrostatic heterodimers are formed in the liquid phase by pairing a zinc porphyrin with either a copper or an aluminium phthalocyanine bearing oppositely charged substitutents. The ground-state absorption spectra of such heterodimers are drastically changed with respect to the corresponding monomers, indicating the existence of a strong interaction between the two chromophores. Irrespective of the nature of the metal ions and of the peripheral substituents, the heterodimers present very similar ground-state spectra.The photophysical properties of the two heterodimers are investigated with the use of femto- and nano-second absorption spectroscopies. Excitation of the zinc porphyrin–aluminium phthalocyanine heterodimer at 565 or 620 nm is followed by a very efficient electron transfer from the porphyrin to the phthalocyanine moiety while a very efficient intersystem conversion takes place in the excited zinc porphyrin–copper phthalocyanine, leading to the final ‘triplet’ excimer. The difference in behaviour is analysed and explained in terms of the thermodynamics of the transfer process and of the peculiar properties of the paramagnetic copper phthalocyanine.

Picosecond generation of transient charge carriers in Langmuir–Blodgett films of semi-amphiphilic heterodimers

Langmuir–Blodgett homolayers are formed by deposition of semi-amphiphilic porphyrin–phthalocyanine heterodimers. The optical and photophysical properties of these dimers have been investigated and compared to the liquid-phase data. Excitation of the dimer results in an instantaneous formation of the singlet excited states, followed by a very efficient charge-transfer reaction. The oxidized porphyrin and reduced phthalocyanine moieties are formed within 2 ps and disappear in 70 ps. The triplet excited states of the porphyrin issued from the intersystem crossing decay pathway of the singlet excited states are formed with a very low quantum yield. They also undergo a charge-transfer reaction, leading to the formation of long-lived transient charge carriers. The photoprocesses determined in the Langmuir–Blodgett films of semi-amphiphilic porphyrin–phthalocyanine heterodimers are almost identical to those previously observed for the same dimers in the liquid phase.

Chemical sensors of monocyclic aromatic hydrocarbons based on sol–gel materials: synthesis, structural characterization and molecular interactions

Low cost materials based on hybrid organic–inorganic sol–gel systems have been developed for use in the trapping and detection of monocyclic aromatic hydrocarbon contaminants emitted into the atmosphere. To this end, two strategies are followed; the first one aims at tailoring the pore size in order to selectively trap and discriminate benzene and toluene. The second is to decrease the pore polarity in order to eliminate the main interfering gas of the atmosphere, water vapor. The results of these strategies are reported in this paper. After describing the synthesis of silicon hybrid xerogels and thin films and the characterisation of their structural properties, we report the study of the local polarity of the pores, of the porosity and optical properties of the materials. These characterisation data will enable us to define the materials suited for hosting benzene and toluene.

Picosecond relaxation of charge transfer states in an asymmetric cerium double-decker sandwich compound

Abstract Subpicosecond time-resolved absorption studies have been carried out on a cerium porphyrin-phthalocyanine sandwich mixed dimer in the liquid and solid states. Following excitation, two relaxation processes having time constants of ≈ 1 and ≈ 40 ps are observed irrespective to the nature of the environment. The first process is attributed to the relaxation of the excited state of the dimer to a low-lying charge transfer state. The second one, much slower, accounts for the relaxation of the charge transfer state back to the ground state. The comparison of the liquid and solid phase data clearly allows the role of the solvent in the relaxation processes to be eliminated.