A. Declémy

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

Picosecond photophysics of diphenylpolyenes: evidence for the influence of excited-state conformational changes on the energy gap ΔE(Bu*-Ag*)

Abstract B u ★ and A g ★ excited singlet states are directly observed in DPH (all-trans-1,6-diphenylhexatriene) and DPO (all-trans-1,8-diphenyloctatetraene) using picosecond transient absorption spectroscopy. Two absorption bands near 4800 and 6500 A are observed and assigned to B u ★ and A g ★ excited singlet states, respectively. Time-resolved evolution, on a picosecond time scale, shows that in both DPO and DPH a thermal equilibrium exists between the B u ★ and A g ★ states with a non-negligible B u ★ population at room temperature even in DPO. Temperature variations enable us to deduce the energy gap Δ E between these two states. It is shown that conformational changes, including the solvent cage effect, significantly influence the Δ E value which is less in the excited-state than in the ground-state conformation.

Effect of ground and excited singlet-state geometry on the level ordering of 1,4-diphenylbutadiene: a theoretical study using MNDO

Abstract Using a MNDO formalism, we have calculated the more stable geometries of the ground and excited singlet states of 1,4-diphenylbutadiene (DPB). We show that the angle between the phenyl plane and the polyene plane is important. In the ground state, the phenyl angle corresponding to equilibrium is near 75°. In the excited B u + state, the corresponding value is near 30°. Our results show that significant conformational change occurs in the excited singlet states. Such a change determines the level ordering of the singlet excited states, and particularly the B u + and A g − excited states.

Solvation dynamics studied by picosecond fluorescence: microscopic reorientation and longitudinal relaxation of the solvent

The dynamics of the Time-Dependent Fluorescence Shift (TDFS) of a rigid polar excited probe dissolved in alcohol solvents at different temperatures have been studied by picosecond time-resolved spectroscopy. The results are compared to previously published results on well characterized polar systems. These results show that solvation dynamics in such systems are strongly scaled by the microscopic (singleparticle) reorientation time τ M of the solvent molecules and/or by the (macroscopic) longitudinal relaxation time τ L of the solvent. The key point governing this scaling is the relative interaction between the solvent molecules and the probe compared to the interaction between the solvent molecules. It is also shown that specific interactions, such as hydrogen bonded-complex formation, may play an important role.

A picosecond transient absorption study of the intramolecular excited state proton transfer in 3-hydroxyflavone: assignment of the observed bands

The time-resolved transient absorption and gain of 3-hydroxyflavone in ethanol ( T = 300 and 173 K) and MCH ( T = 300 and 198 K) has been investigated. Two absorption bands (A and B) and two gain bands (E1 and E2) have been observed. Band A (lying in the 5200 A spectral region) appears during and just after the excitation step and is assigned to the normal excited form N★x Band B (lying in the 6050 A spectral region) appears after band A and is assigned to the tautomer form T★. A gain band E2 (lying at λ=5400 A) appears at the same time as the B band and is attributed to the T★→T emission. A gain band E1 (lying at λ = 5730 A) appears earlier than the E2 band. Its origin is discussed and two possible assignments are presented: vibronic progression of the emission spectrum of T★ or formation of an intermediate species on the proton transfer pathway from N★x to T★.

Picosecond spectroscopic investigation of the internal conversion rate of excited perylene in solution

Abstract The dynamics of relation from upper vibrational levels of the S 1 * state and from the upper vibronic states S 3 * and S 4 * of perylene dissolved in organic solvents of various viscosities and polarities have been investigated. Unexpected slow decay times have been measured, of the order of 30 ps for vibrational relaxation in the S 1 * manifold and of the order of 50–60 ps for internal conversion from states S 3 * and S 4 *. Significant solvent effects have not been observed on changing the polarity and the viscosity of the solvent.

Totally automated picosecond transient absorption spectrometer using two photodiode arrays as detectors

A totally automated picosecond transient absorption spectrometer is described. This experimental apparatus requires two photodiodes arrays as detectors and a single spectrograph. It takes advantage of the high dynamic range of such detectors. Typical transient spectra are presented. They show the reproducibility and accuracy of this method of time‐resolved spectroscopy.

Relaxation dynamics of the solvent cage around an excited molecule in solution

This paper deals with the relaxation dynamics of a polar solvent cage surrounding a polar excited probe. The dynamics of the time‐dependent fluorescence shift (TDFS) of a polar, rigid probe dissolved in polar protic and aprotic solvents at different temperatures is studied by time‐resolved spectroscopy. The results are compared to recent or previously published results on well characterized polar systems. These results show that solvation dynamics is closely related to the microscopic reorientation time (τM) of the solvent molecules and/or to the (macroscopic) longitudinal relation time (τL) of the solvent. The key point governing this relation is the strength of interactions between the solvent molecules and the probe compared to the interactions between the solvent molecules themselves. It is also shown that specific interactions, such as hydrogen‐bonding may play an important role in the TDFS and cage relaxation behavior.