G. W. Robinson

Direct observation of rotational diffusion by picosecond spectroscopy

Abstract Time dependent fluorescence depolarization measurements have for the first time been extended to the picosecond regime by using streak camera optical multichannel analyzer detection. Fluorescein derivative dyes (M.W. 500-1000) rotate in polar solvents as if their volume is at least double that of the free molecule because of solvent attachment, an effect noted by Einstein and elaborated upon by Marinesco and Perrin many years ago. This effect is apparently the major cause of the breakdown of the literal Einstein hydrodynamic model for rotation of these relatively small molecules in solution. The solvent attachment also very likely reduces the effect of molecular shape on rotational depolarization, causing these molecules to behave more like spheres than their molecular structure would imply. Both the fluorescence decay curve and the rotational correlation function derived from the experimental data, within the limits of experimental error, decay as pure exponentials. In addition, unlike most fluorescence probe experiments on nanosecond time scales applied to the study of macromolecular structure, the theoretical value of 0.4 for the polarization anisotropy at zero time is observed. Comparison of these types of results on picosecond time scales could have implications in the study of flexions and rotations of the substructure of large molecules having biological importance.

Time dependent fluorescence depolarization studies of BBOT

Abstract Picosecond time dependent fluorescence depolarization has been used to study the rotational diffusion of 2,5-bis (5′-t-butyl-2-benzoxazylyl) thiophene (BBOT) in cyclohexane and ethanol. The measured rotational correlation times were: 180 ± 30 ps (cyclohexane) and 210 ± 30 ps (ethanol). These results are in agreement with those predicted from hydrodynamics with the slipping boundary condition.

Rotational diffusion of the mode-locking dye dodci and its photoisomer

Picosecond time dependent fluorescence depolarization has been used to measure the rotational diffusion time of 3,3′-diethyloxadicarbocyanine iodide (DODCI) in solution at 293 K: the stable form of DODCI was shown to have a “coiled” form, the cis,cis-1,5-conformation, whereas the photoisomer formed by irradiating DODCI in solution was shown to be the elongated all-trans-isomer. The stable form has a rotational correlation time of 160 ± 30 ps in ethanol and 320 ± 40 ps in isopropanol; the fluorescence lifetimes were 1.2 ± 0.1 ns (ethanol) and 1.6 ± 0.1 ns (isopropanol). The all-trans-photoisomer has a rotational correlation of > 700 ps in isopropanol and a fluorescence lifetime of 0.42 ± 0.05 ns. Furthermore, the rotational diffusion times agree closely with those predicted on the basis of the Stokes-Einstein hydrodynamic model, indicating a lack of any solvent attachment to DODCI in solution.

PICOSECOND SPECTROSCOPIC STUDIES OF SPONTANEOUS AND STIMULATED EMISSION IN ORGANIC DYE MOLECULES

Abstract Stimulated emission from organic dyes provides us with the potential for producing tunable, short pulsed lasers. Stimulated emission can also interfere in the measurement of excited state lifetimes of dyes when excited by intense picosecond pulses. Time-resolved measurements of the emission from several organic dyes as a function of excitation intensity have been carried out in order to determine the conditions for obtaining efficient dye lasers and correct excited state lifetimes. A long build-up time for the stimulated emission (≈) 60 ps for a 6–8 ps exciting pulse) has been observed and confirmed by a theoretical model. Calculations using this model show that, even in the presence of very weak stimulated emission, the emission intensity does not decay in the same manner as the density of excited molecules.

Exciton fission and annihilation in crystalline tetracene

The time-resolved fluorescence of tetracene crystals excited by a single 7-ps pulse of 530-nm light has been measured with a streak camera/OMA system. The singlet exciton lifetime was found to be 300±30 ps at 293 K. Singlet-singlet exciton annihilation was observed, and the rate constant found to be 5 x 10-9 cm3 s-1. In order to fit the observed decay curves, an upper limit of c. 1 x 10-9 cm3 s-1 must be set on the singlet-triplet exciton annihilation rate. The measured singlet-singlet exciton annihilation rate is consistent with a hopping model for singlet exciton motion at room temperature.

Chemically stiff water: Ions, surfaces, pores, bubbles and biology

Abstract At water/air interfaces or in molecular contact with hydrophilic or hydrophobic surfaces, interfacial water separates the bulk from the adjoining phase. Water near dissolved molecules has interfacial properties, and many fundamental biological processes occur in so small a volume that all the water involved must be interfacial. In this paper, the stiffness of interfacial water is used to study its properties.

Temperature dependence of proton recombination and proton-induced quenching for 2-naphtholate

Lifetime and quantum yield measurements in aqueous solutions have been performed on the anion (RO*/sup -/) of electronically excited 2-naphthol (ROH*). Measurements were made at moderately low pH in order to determine the temperature dependence of the rate of proton recombination, k/sub rec/, with this anion. As predicted by recently proposed absolute rate expression for weak acid dissociation/recombination, k/sub rec//sup -1/ has the same temperature dependence and magnitude as the Debye (transverse) rotational relaxation time of the pure water solvent. It is also likely that the rate for the proton-quenching reaction, H/sup +/ + RO*/sup -/ ..-->.. ROH (ground state), has the same temperature dependence as k/sub rec/. These types of experiments are of great importance toward the fundamental understanding of ion-hydration reactions, since up to now purely theoretical considerations have indicated that it is the much shorter longitudinal relaxation time that sets the time scale for ion solvation in water.

Cooperativity in liquid water

Hydrogen-bond cooperativity in water is reflected by temperature-dependent potential barriers hindering the rotation of individual molecules. These barriers give rise to temperature-dependent activation energies for relaxation phenomena and temperature-dependent librational frequencies. This paper discusses the origin of these barriers and relates them to H2O and D2O liquid densities, ≈ 240–400 K.

THEORY OF ACTIVATED RATE PROCESSES WITH SPACE-DEPENDENT FRICTION

Abstract We generalize the Carmeli—Nitzan unified theory for the non-Markovian escape rate from a metastable well by allowing for different memory friction kernels in the sell and the barrier regions. These rates, which contain previously known results as special cases, are plotted for various values of the relevant parameters.