C. Rullière

Laser action and photoisomerisation of 3,3′-diethyl oxadicarbocyanine iodide (DODCI): Influence of temperature and concentration

Abstract Formation and decay rate constants of the DODCI photoisomer are found to follow an Arrhenius law with respectively an activation energy of 4.5 ± 0.4 kcal/M and 14 ± 0.3 kcal/M. On the basis of these results a kinetic scheme is proposed for DODCI photoisomerisation. It appears from measurements of the fluorescence quantum yield variation as a function of the temperature that non-radiative transitions in DODCI are principally due to photoisomerisation. DODCI wavelength laser emission changes with the temperature showing that above −5°C laser action is due to the photoisomer and below −40°C to DODCI normal form. Between −5°C and −40°C laser action occurs simultaneously from DODCI photoisomer and normal form at two different wavelengths. For a given excitation energy, variation of DODCI concentration is shown to induce a large change in the spectral distribution of DODCI fluorescence. This implies that an important radiative energy transfer does occur between DODCI normal form and the photoisomer.

Picosecond absorption spectra of carbonyl derivatives of dimethylaniline: the nature of the tict excited states

Abstract Distinct picosecond absorption bands around 500 nm appearing with a delay following the excitation of several carbonyl derivatives of N,N-dimethylaniline in polar solvents show similarities to the spectra of radical anions of the acceptor moiety of the molecule. The rise and decay of these bands correspond to those of the “anomalous” fluorescence. The spectra show that the emitting state is a twisted intramolecular charge transfer (TICT) excited state.

Wide-field optical coherence tomography: imaging of biological tissues

We describe a two-dimensional optical coherence tomography technique with which we were able to obtain multiple longitudinal slices of a biological sample directly in a single Z scan. The system is based on a femtosecond Cr4+:forsterite laser and an infrared camera for wide-field imaging of the sample with a depth resolution of 5 microm. With this imaging apparatus we were able to investigate human skin and mouse ear samples and to observe the different constitutive tissues.

Dual fluorescence in trans-4-dimethylamino-4'-cyanostilbene revealed by picosecond time-resolved spectroscopy: a possible new tict compound

Abstract The presence of two time-resolved emission bands is observed in trans-4-dimethylamino-4′-cyanostilbene dissolved in polar solvents using picosecond time-resolved spectroscopy. In a non-polar solvent, only the presence of a single emission band is observed at any temperature. A precursor-successor relationship occurs between the two species responsible for the two emission bands. It is concluded that the formation of a twisted intramolecular charge transfer (TICT) exicted state may explain these experimental results.

Photophysics of 4-dimethylamino 4′-cyanostilbene and model compounds: dual excited states revealed by sub-picosecond transient absorption and Kerr ellipsometry

4-Dimethylamino 4′-cyanostilbene (DCS) and two selectively bridged compounds are investigated using sub-picosecond time-resolved absorption and Kerr ellipsometry experiments. The latter technique makes it possible to work at low excitation energy and low concentrations, thereby avoiding intensity and concentration effects which exist in DCS derivatives. Using this technique, in a non-polar solvent (cyclohexane), only the presence of a single excited state is observed for all studied compounds. In polar solvents, the bridged derivative where twisting of the anilino moiety is prevented but double bond twisting is allowed also reveals the presence of only one excited state. On the other hand, in polar solvents, for the DCS compound and the related bridged derivative where the anilino moiety is still able to twist, a precursor-successor relationship is clearly observed between two different excited states. These results can be understood within a four excited states model derived from a previously suggested diagram: the DE state (delocalized excited state), the ICT state (internal charge transfer, highly polar, nearly planar configuration, formed quasi-instantaneously from the DE state by electronic reorganization), the CRICT state (conformational relaxed ICT state, highly polar and fluorescent, involves conformational geometric changes such as twisting of the anilino group which enhances charge transfer) and the “phantom” state P∗ on the trans⇄cis isomerization pathway (twisted double bond, low polar, non-fluorescent). In this study, we observed the formation of the CRICT excited state on a time scale ranging from 2 to 20 ps depending on the solvent characteristics (polarity, viscosity and hydrogen bonding ability).

Picosecond solvation dynamics: the role of the solvent microscopic relaxation time in highly polar aprotic solvents

Abstract The solvation dynamics of a rigid polar probe dissolved in highly polar aprotic solvents has been studied using time-resolved fluorescence techniques. We have observed good correlation between the characteristic solvation time τ R and the microscopic relaxation time τ M of the individual solvent molecules. Near the probe the “continuum” description of the solvent fails and microscopic description is necessary.

Picosecond solvation of electronically excited solutes in alcoholic solvents: non-debye behaviour of the time-dependent fluorescence shift related to h

Time-dependent fluorescence shifts (TDFS) of polar compounds in a series of alcoholic solvents have been studied on a pico-second time scale. A non-exponential TDFS kinetic behaviour is observed, particularly at short times (0<t< 50 ps). It cannot be represented by a sum of decreasing exponentials. The effects of solute-solvent interactions on the kinetics of TDFS have been analysed. It is shown that the non-standard kinetics is due to hydrogen bonding in the solvent, particularly to the presence of alcoholic hydrogen-bonded aggregates. Following electronic excitation of the polar solute reorganization of solvent aggregates controls the TDFS time development. Our results show that a simple Debye model is inadequate to explain such a process.

Picosecond hydrogen-bond formation in excited states: The case of an intramolecular charge-transfer (ICT) state

Abstract Time-resolved emission spectra of 7-amino-3-methyl-1,4-benzoxazine-2-one (AMBO) are plotted in the time range 0

Time-resolved dual fluorescence of push—pull stilbenes at high solute concentration and excitation intensity: evidence for an emitting bicimer

Abstract Time-resolved emission spectroscoy of “push—pull” stilbene derivatives has revealed the presence of an anomalous emission band appearing in the 5200–5600 A spectral range. This anomalous band appears only in derivatives where the donor group is able to twist and depends on the solvent polarity, concentration and intensity excitation. This band, correlated to the presence of excited TICT states, is attributed to the emission of a “bicimer” species formed by the association of a pair of electronically excited state with large dipole moments.

Real-time two-dimensional imaging in scattering media by use of a femtosecond Cr 4+ :forsterite laser

An original femtosecond Cr(4+):forsterite laser source associated with a nonlinear optical correlator was used for imaging through scattering media with 1220-nm light. The system, which operates as an ultrafast optical gate by sum-frequency generation in a nonlinear crystal, was able to detect the light reflected from a resolution chart hidden in a turbid medium, at an attenuation of as much as 15 mean free paths. When the object was illuminated with a collimated beam, real-time two-dimensional images were obtained, with a maximum transverse resolution of ~20 microm.