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Fluorescence spectroscopy and isothermal titration calorimetry (ITC) have been used to study the interactions in water at 25 degrees C of two anionic surfactants--sodium dodecyl sulfate (SDS) and sodium perfluorononanoate (SPFN)--with various pyrene-labeled hydrophobically modified poly(N-isopropylacrylamides) (HM-PNIPAM) grafted at random with small amounts of fluorocarbon chains (1H,1H-perfluorooctyl, CH2C7F15); (PNIPAM-F), or (n-octadecyl, C18H37) (PNIPAM-HPy) or both (PNIPAM-F/HPy). In aqueous solution, the copolymers form micellar structures consisting of a loose corona of hydrated poly(N-isopropylacrylamide) chains and a hydrophobic core rich in hydrocarbon or fluorocarbon groups. From fluorescence studies based on changes in the ratio of pyrene excimer to monomer emission intensity, it has been established (1) that mixed SDS/C18H37 clusters form along the polymer chain upon addition of SDS to either PNIPAM-HPy or PNIPAM-F/HPy and (2) that SPFN does not interact with the hydrocarbon-rich microdomains of the polymeric micelles. The conclusions were corroborated by ITC experiments, which yield the overall enthalpy change associated with polymer/surfactant interactions. They provided strong evidence (1) that SDS molecules adsorb along the PNIPAM main chain but do not mix with the fluorocarbon-rich microdomains of PNIPAM-F or PNIPAM-F/HPy and (2) that SPFN associates with the perfluorocarbon substituents of PNIPAM-F and PNIPAM-F/HPy but has a poor affinity for the polymer chain.

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 prepar...

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.

Ground and excited state properties of a pre-twisted 7-diethylaminocoumarin dye (BC I) belonging to the family of coumarinyl benzopyrano pyridines are reported in isooctane-AOT-formamide reversed micelles. In reversed micelles, BC I, albeit soluble in formamide, is found to remain out of the polar solvent pool. But the photophysical properties of the probe dye are sensitive to the changes in the polarity of the interfacial region caused by increase in F0 = [formamide]/[AOT]. The spectroscopic properties and dynamics are indicative of dual emission due to the solubilization of the dye in two different environments (the nonpolar solvent and the interfacial region). Results of the steady-state fluorescence anisotropy experiments also support the presence of two different environments. The present study once again proves that molecular size is an important parameter in the study of the photophysical properties of the flexible aminocoumarin dyes in reversed micelles.

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 observat...

Electronic absorption spectrum in the near IR region (Qy band) of LH I of Rhodopsirillum rubrum and LH 11 of Rhodopseudomonas acidophila is calculated using an electrodynamical point dipole approximation. The model uses the geometrical arrangement and the properties of individual chromophores made available from recent structural and spectroscopic determinations and is found to accurately predict not only the resonance frequencies, but also the shape of the Qy band of the absorption spectrum.

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 W0 (=[water]/[AOT]) values (0 < W0 < 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 W0 (≥10) 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 m...

Time resolved emission spectroscopy of a water insoluble aminocoumarin derivative, BC I, in n-heptane–AOT–water reversed micelles is investigated using a nanosecond fluorescence spectrometer. The extent of the time-dependent Stokes shift is found to be smaller for W0=4 than for W0=10 and also for W0=40 (W0, molar ratio of water and AOT). A monoexponential decay of the dynamic Stokes shift correlation function with a time constant of 4.5 ns is observed at W0=4. At W0=10 the time-correlation function shows a bi-exponential decay with a fast component of ∽600 ps and another of 3.3 ns comparable to the fluorescence lifetime of the dye. The time correlation function at W0=40 shows an extremely slow component of the solvation time which is at least an order of magnitude larger than the fluorescence lifetime of the probe dye. In addition a faster component with a solvation time of 2.15 ns is also observed. The solvation time observed is likely due to the diffusion of the probe dye within the micellar interface or due to the reorganization of the water molecules bound to the polar head groups of AOT. Steady-state absorption and fluorescence, and time resolved fluorescence study of another aminocoumarin, C 480, give evidence of equilibrium between the distribution of the dye molecules between the interface and water pool. The time resolved shift of the emission spectra is more likely due to the diffusion of the probe between the interface and water pool rather than reorientation of the water molecules (solvent relaxation) in the core of the reversed micelles as it has been reported earlier.

The effect of water on the photophysical properties of four 7-diethylaminocoumarin dyes (AC 1, BC I, BC II and C 35) in dioxane–water solvent mixtures is studied. Dielectric enrichment of the dyes is not observed in the binary mixtures. Rate constants for bimolecular quenching are calculated. BC I and C 35 are found to undergo both static and dynamic quenching while only static quenching is observed in the case of BC II. The association constant is however found to be very small (Kb=2–5×10-2 M-1). No such dependence is observed in AC 1 where the formation of an intramolecular hydrogen bond is strongly favored. The increase in the non-radiative rate constant with the water concentration observed in the other three dyes (BC I, BC II and C 35) can be correlated to the solvent polarity and hydrogen bonding parameter ET(30). A threshold value of the solvent polarity–polarizability parameter (Pekar factor C) for the non-radiative process is observed for all these three dyes and it is nearly independent of the nature of substitution. The photophysical properties of AC 1 can be adequately explained by considering only one singlet excited state while an additional singlet excited state needs to be included to understand the photophysics of BC I, BC II and C 35 in highly polar environments.

— In this short communication we present the stoichiometric ratio of bacteriochlorophyll, bacteriopheophytin and carotenoids in a few photosynthetic purple bacteria complexes (whose two-dimensional or three-dimensional structures are well known) determined using the spectrum-reconstruction method (SRCM). An important conclusion of our pigment stoichiometric analysis is the evidence for the absence of the second carotenoid in the light-harvesting complex 2 (LH2). In the process, we also highlight the useful application of SRCM in determining the molar extinction coefficients of carotenoids present in LH1, LH2 or reaction centers for which these values are not known due to isolation problems and/or stability.

A detailed investigation of the photophysical properties of a new coumarin heterodimer: 2-(7-diethylamino-3-coumarinyl)-5-oxo-4-methyl [1] benzopyrano(3,4- c )pyridine (CBP) has been performed in a range of nonpolar and polar solvents at room temperature. In addition, the influence of temperature on the excited state properties of CBP in dichloromethane, methanol and dimethylsulfoxide has also been investigated. These studies indicate that the fluorescence of the dye is from the pre-twisted intramolecular charge transfer (ICT) state. AM1 semiempirical calculations performed for CBP in the ground, excited singlet and triplet states are in conformity with the experimental results. The presence of second coumarin is found to introduce a larger charge transfer character to the parent coumarin molecule when compared to other diethylamino coumarins. From this study it is also evident that such coumarin hetero dimers behave more like any other single absorbing and emitting chromophoric-systems.

Photophysical properties of two selectively substituted bicoumarins bridged by a flexible single covalent bond have been investigated in solvents of varying polarity. The results show that the seco...

The absorption and fluorescence characteristics of four 7-diethylamino-3-styryl coumarins (dyes I–IV), containing either an extended formyl or acetyl substituent at the 3-position, in solvents of different polarity (chloroform, dimethylsulphoxide and dimethylformamide) were studied with regard to the influence of the flexibility of the chromophoric system in both the ground and excited states. The photophysical measurements show that the dyes are very weakly fluorescent. The fluorescence lifetimes are in the region of 0.01 ns and the non-radiative rate constant is very high when compared with the radiative rate constant. These observations indicate that the reduction of the acceptor strength of the substituent at the 3-position, as in the present case, in a 7-diethylaminocoumarin dye does not always result in sustained fluorescence in polar, aprotic solvents. Therefore these days are not technically important as laser dyes.

Photophysical properties and Energy transfer Dye Laser (ETDL) characteristics of a new dye: 7-DiethylAmino Coumarin with a Heteroaryl substitution in the 3-position (DACH), are presented. The suitability of DACH as a laser dye has been verified on the basis of the absorption and fluorescence characteristics in aprotic solvents. The dye laser characteristics of the dye pumped by the available nitrogen laser have been successfully investigated in dimethylsulfoxide by energy transfer dye laser mechanism, using coumarin 1 as a donor dye. DACH has been found to lase in the blue-green region of the visible spectrum. The experimental and theoretical values of energy transfer rate constant (kET) and the critical transfer distance (R0) have been calculated in two different solvents to determine the nature of energy transfer. The results indicate the dominance of resonance transfer in this ETDL.

The photophysical properties of a new dye, 7-Diethyl AminoCoumarin with a Rigid substitution in the 3-position (referred to as DARC) have been studied in three solvents: dioxane, DMF and DMSO. The dye has been found to have a fluorescence quantum efficiency (ϕfl) between 0.40 and 0.80 in these solvents. The dye-laser performance of this dye has also been investigated in dioxane, DMF and DMSO, under nitrogen-laser pumping and compared with that of the commercially available standard laser dye, Coumarin 515 (C-515). A tuning range of nearly 70 nm was obtained in the blue-green region with an efficiency up to 80% of that of the standard dye. The observed characteristics of the dye are explained in terms of the sructural rigidization of the dye in the 3-position which inhibits the formation of the Twisted Intramolecular Charge Transfer (TICT) conformation in the excited state leading to an enhancement of theϕfl and a considerable improvement in the laser performance.

An experimental study of the laser characteristics, by energy transfer machanism of a new dye: 7-Diethylamino-3-Cyano Benzimidazo (1,2-a) Quinoline (DCBQ) using 7-Diethylamiho-4-Methyl Coumarin (Coumarin 1,DAMC) as a donor are presented. The effect of the concentration of the components on the lasing characteristics like peak power and wavelength of lasing of dye mixture is studied and discussed. The effect of the solvents on the fluorescence energy transfer has also been studied to ascertain the nature of the transfer mechanism involved. The fluorescence energy transfer rate constant(kET) and the critical transfer distance(Ro) are experimentally determined and also calculatee theoretically. These results indicate the dominance of resonance transfer in this energy transfer dye laser.

Various photophysical parameters of three substituted 7-diethylamino benzimidazo quinolines (I, II, and III) have been evaluated in three solvents: DMSO, DMF and chloroform. Based on these parameters, the probability of lasing action by the dyes is predicted and is confirmed from the preliminary laser investigations. Dye I (-CN Substitution in 3-position) and Dye III, which has an extended conjugated system in 3-position , have been found to be potential laser dyes, while Dye II (-CHO substitution in 3-position) is not.

The laser characteristics of a new dye, 7-Diethyl-amino-3-Styryl Benz31 yimidazo (1,2-a) Quinoline (DSBQ) have been investigated in two solvents: DiMethylSulfOxide (DMSO) and DiMethylFormamide (DMF) by excitation with a pulsed nitrogen laser and compared with that of the standard dye rhodamine B. The lasing range of the dye is nearly 40 nm with maxima at 635 nm and 625 nm in DMSO and DMF, respectively. The efficiency of this dye (DSBQ) has been found to be better than half that of rhodamine B.