The spectroscopic properties and dynamic behavior of peridinin in several different solvents were studied by steady-state absorption, fluorescence, and transient optical spectroscopy. The lifetime of the lowest excited singlet state of peridinin is found to be strongly dependent on solvent polarity and ranges from 7 ps in the strongly polar solvent trifluoroethanol to 172 ps in the nonpolar solvents cyclohexane and benzene. The lifetimes show no obvious correlation with solvent polarizability, and hydrogen bonding of the solvent molecules to peridinin is not an important factor in determining the dynamic behavior of the lowest excited singlet state. The wavelengths of emission maxima, the quantum yields of fluorescence, and the transient absorption spectra are also affected by the solvent environment. A model consistent with the data and supported by preliminary semiempirical calculations invokes the presence of a charge transfer state in the excited state manifold of peridinin to account for the observations. The charge transfer state most probably results from the presence of the lactone ring in the π-electron conjugation of peridinin analogous to previous findings on aminocoumarins and related compounds. The behavior of peridinin reported here is highly unusual for carotenoids, which generally show little dependence of the spectral properties and lifetimes of the lowest excited singlet state on the solvent environment.

The dissolution in water of the sodium salts of poly(2-acrylamido)-2-methylpropanesulfonic acid (PAMPS), N,N-di-n-octadecyl-endcapped-poly(2-acrylamido)-2-methylpropanesulfonic acid [(C18)2-PAMPS], cholesteryl-endcapped-poly(2-acrylamido)-2-methylpropanesulfonic acid (Chol-PAMPS), and a random copolymer of AMPS and N-dodecylmethacrylamide (PAMPS-Dod20) was studied by isothermal titration calorimetry (ITC). The endcapped polymers form multimolecular aggregates in aqueous solutions. The concentration range for aggregation and the enthalpy of micellization were determined as functions of the electrolyte concentration (0.01 M < [NaCl] < 0.3 M) and temperature (288 K < T < 308 K) for solutions of (C18)2-PAMPS. At 298 K and 0.2 M NaCl, aggregation of this amphiphilic polyelectrolyte occurs in solutions with concentrations ranging from about 1 to 14 mmol of AMPS L-1 (0.5−2.7 g L-1) with an enthalpy of micellization of ∼100 J (mol of AMPS)-1. The effect of the experimental conditions, such as the method of preparation of the polymer stock solution and its concentration, on the ITC results is described in the case of Chol-PAMPS. Titrations performed with poly(sodium-2-acrylamido-2-methylpropane sulfonate) (PAMPS) confirmed that PAMPS does not aggregate in aqueous NaCl, whereas PAMPS-Dod20 forms predominantly unimolecular micelles. The advantages and limitations of ITC for studying the micellization thermodynamics of amphiphilic polyelectrolytes are discussed.

Photophysical properties of two 7-diethylaminocoumarin dyes belonging to the family of coumarinylbenzopyrano pyridines (BC I and BC II), and a third dye, 7-diethylamino-4-trifluoromethyl coumarin (C 35), are reported in Aerosol OT (AOT) and water-in-oil (n-heptane/AOT/water) microemulsions. In the case of BC I, emission arising from two different environments for the dye is observed. Results of the steady-state fluorescence anisotropy experiments also support the presence of two different environments. At low W 0 ()[water]/[AOT]) values (0 < W 0 < 10), the excited-state properties of the molecule in the interfacial region are found to be sensitive to the polarity of the surrounding microenvironment. The decrease in the fluorescence quantum yield at higher W 0 (g10) values can be attributed to the intermolecular hydrogen bonding between the cyano (-CN) group of the dye and water molecules present in the micellar interface. These observations are explained on the basis of the comparable size of the probe dye molecule and the molecular dimensions of the interfacial region in the microemulsions. The large molecular size of BC II is also found to be the reason for the sensitivity of the dye to the microenvironment of n-heptane/AOT/water microemulsions. However, the effects are much smaller than those observed in BC I. A similar photophysical study involving C 35 is found to show no such effects. The smaller molecular size, charge neutrality, hydrophobic nature, and a preferential solvation in heptane are essentially the reasons for the results observed in C 35.

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Steady-state fluorescence spectroscopy and isothermal titration calorimetry (ITC) have been used to study the aggregation in aqueous solutions of poly-(2-acrylamido)-2-methylpropanesulfonic acids, sodium salt mono-endcapped with either N,N-di-n-octadecyl or N-4-[(1-pyrenyl)butyl]-N-n-octadecyl which were prepared by free radical polymerization of 2-(acrylamido)-2-methylpropane sulfonic acid (AMPS) initiated with the azo compounds, 4,4'-azobis{cyano-N,N-di-n-octadecyl}pentanamide and 4,4'-azobis{cyano-N,N-[4-(1-pyrenyl)butyl]-n-octadecyl}pentanamide, respectively. Both techniques indicate the occurrence of multimolecular aggregates in solutions of the polymers in water and in 0.2 M NaCl. The concentration range for aggregation is about 1-14 mmol AMPS l −1 (0.5-2.7 g l −1 ) in 0.2 M NaCl and the enthalpy of micellization, estimated from ITC data, is ∼100 J [mol AMPS] −1 . The accessibility of the chromophores to neutral molecules and to cationic species was assessed by quenching of fluorescence with nitromethane and thallium nitrate, respectively. The association of the mono-endcapped polymers is compared to that of PAMPS derivatives carrying hydrophobic groups randomly attached along the chain.