P. Plaza

Transient absorption of symmetrical carbocyanines

Abstract Transient absorption in several symmetrical cyanine and carbncyanine dyes is measured by time-resolved spectroscopy with subpicosecond white-light continuum in a wide spectral range (0.35–1 μm). The results are compared with data available in the literature on ultrashort and long-lived transients. The linear shift law observed here for the S1Sn absorption bands in the wavelength scale allows one to predict the main induced absorption bands for members of several carbocyanine families (vinylogous series) which have not yet been studied. In the case of the shortest methine chains (cyanines and monocarbocyanines) a tew transient absorption bands are assigned to the formation of photoisomers. The absolute values of the excited-state or photoisomer cross sections of polymethine carbocyanines are evaluated by global spectral analysis, from the measured differential optical density spectra. Several cases of large values (higher than 10) of the absorption cross section ratio (excited/ground state) are found at new wavelengths, which allows applications of photo-induced absorption to photonics and laser technologies over a broad spectral range.

Subpicosecond reverse saturable absorption in organic and organometallic solutions

Solutions of several carbocyanines, phthalocyanines and naphthalocyanines were studied by time-resolved transient spectroscopy with subpicosecond white-light continuum. The excited-state absorption cross-sections of all compounds were determined from the differential spectra obtained at short delay time after excitation, by a global spectral analysis. All these molecules exhibit an excited-state absorption cross-section higher than that of the ground state at 610 nm. This spectral property gives rise to a reverse saturable absorption (RSA) effect under high laser fluences at this wavelength. Nonlinear transmission of these molecules was measured under increasing laser fluences. A three-level molecular model was used to simulate the measured nonlinear transmission and the best fits were obtained with molecular parameters in good agreement with those deduced from the analysis of the transient spectra. The use of RSA for the energy stabilization of ultrashort laser pulses was demonstrated, and a stabilization coefficient was defined as an efficient tool for characterizing nonlinear behavior of such compounds.

Investigation of excited-state charge transfer with structural change in compounds containing anilino subunits by subpicosecond spectroscopy

Subpicosecond absorption spectroscopy and picosecond streak camera fluorometry are applied to the study of the photoinduced processes in three classes of compounds for which molecular torsion with charge localization leading to an emissive or non-emissive twisted internal charge transfer state was proposed. The compounds are classified according to their steady-state fluorescence properties: compounds having a single band without solvatochromism or a single band with solvatochromism and compounds exhibiting dual fluorescence in polar solvents. The selected compounds are di- and tri-phenylmethane and amino-rhodamine dyes for the first class, the laser dye DCM for the second class and a substituted triphenylphosphine for the third class. These compounds contain at least one electron-donating anilino-group able to rotate. In all cases, the transient spectroscopy gives evidence for an excited-state relaxation involving the formation of an intermediate state. The changes observed in the kinetics by changing the solvent viscosity and polarity and upon donor protonation bring support to a mechanism involving photoinduced charge transfer with structural change. The kinetics are tentatively discussed within the context of theories of electron transfer reactions in the condensed phase.

Ultrafast excited singlet state absorption/gain spectroscopy of perylene in solution

Abstract Time-resolved absorption and gain spectroscopy of perylene in the lowest excited singlet state S 1 in solution is performed in the spectral range 350–1000 nm using subpicosecond excitation at 425 nm. Strong S 1 S n absorption at 700 nm ( e = 53000 M −1 cm −1 ) as well as the full S 1 S 0 stimulated emission cross section spectra are measured. A way to extract, from the sample absorbance variation, the S 1 population, and the absolute value of the transition cross sections, is given for the perylene case. Direct evidence of fast vibrational relaxation in about 1 ps is given for the 352 cm −1 mode in the S 1 state. It is suggested that this molecule could be a good choice as a reference for the measurement of the excited state absorption cross sections.

Ultrafast internal charge transfer in a donor-modified rhodamine

Abstract A rhodamine dye ARh with a 4-dimethylamino (electron donor) instead of the usual 2-carboxylate (electron acceptor) substituent is described and its photophysical properties are rationalized within the twisted internal charge transfer (“TICT”) state model. ARh is nearly non-fluorescent in fluid solution but can be transformed into a highly fluorescent dye by protonation. This intramolecular fluorescence quenching in fluid neutral solutions is characterized by stationary and time-resolved fluorescence measurements and the product state is evidenced by subpicosecond transient absorption and gain spectroscopy.

Transient spectroscopy of triphenylmethane derivatives following subpicosecond irradiation

Time-resolved absorption spectra of triphenylmethane dye solutions are measured in the 320–750 nm wavelength range in pump-probe experiments with subpicosecoud resolution. The observed spectral evolution occurs at a rate which depends on solvent viscosity. The transient spectra are described by the superposition of the absorption and/or gain spectra of the states S0 and S1, and of an intermediate state Sx involved in the relaxation process. The results indicate that the Sx absorption spectrum is similar to that of S1 but blue-shifted.

Fluorescence quenching of auramine in fluid solution: a femtosecond spectroscopic study.

The quenching of Auramine fluorescence in ethanol is studied by two ultrafast spectroscopy techniques. The gain band, probed by transient absorption spectroscopy, vanishes in a few picoseconds, while a transient absorption band rises and the ground-state repopulation is delayed. In up-conversion experiments, nonexponential wavelength-dependent fluorescence decays are observed. The average decay times increase with the wavelength and the reconstructed instantaneous spectrum exhibits a few hundred-wavenumber red shift and a broadening while its intensity drops. The previously proposed relaxation model, involving a barrierless internal twisting motion toward a transient dark state, is further examined. In particular, the extinction coefficients of the transient state are extracted from the differential absorption spectra. The band is found to lie in the same wavelength range as the dimethylaniline cation radical. This result is discussed as a possible support for an internal twisting process involving a charge shift.

PHOTOPHYSICS OF TRIPHENYLPHOSPHINES AND THEIR OXIDES : ROLE OF DIMETHYLAMINO SUBSTITUENTS

Abstract Steady-state and time-resolved spectroscopy experiments with picosecond or subpicosecond resolution are carried out in order to clarify the anomalous fluorescence properties of solutions prepared with triphenylphosphine compounds in various non-deaerated solvents at room temperature. Evidence is given for the formation of triphenylphosphine oxides in the solutions and for the coexistence of the oxidized and non-oxidized forms at the concentrations used for time-resolved experiments. The photoinduced formation of an emissive charge-transfer state, previously reported for solutions prepared with the dimethylamino substituted triphenylphosphines, is shown to be due to the corresponding oxide which exhibits a dual fluorescence with large solvatochromism. The non-oxidized forms of both the dimethylamino substituted and unsubstituted triphenylphosphines are found to exhibit a strongly red-shifted emission band with little solvatochromism. A photoinduced geometrical change with an increase of the conjugation in the excited state is proposed to explain this observation. On the other hand, an extra fluorescence band occuring at shorter wavelengths for the non-oxidized dimethylamino-substituted compound is not completely understood.

A new concept of photogeneration of cations: Evidence for photoejection of Ca2+ and Li+ from complexes with a crown-ether-linked merocyanine by picosecond spectroscopy

Time-resolved transient absorption and gain spectra of DCM-crown and its complexes with Li+ and Ca2+ in acetonitrile are measured in the wavelength range 370–670 nm after subpicosecond excitation at 425 nm. The results give evidence for the fast formation of free DCM-crown from both excited complexes, with a faster rate for the Li+ complex. A two-step mechanism is found for the initial decay (within 30 ps) of the free DCM-crown.

Fast Change in the Excited State Absorption Spectrum of TPM Dye Solutions

Time evolution of the transient absorption spectra of a TPM dye is measured in pump-probe experiments using an unconventional subpicosecond tunable dye laser system. New transient absorption bands in the blue are reported.