Isoemissive Points Research Articles

Exploring cationic polyelectrolyte-micelle interaction via excited-state proton transfer. Signatures of probe transfer.

Excited-state proton transfer (ESPT) is a sensitive tool for the delicate monitoring of structural reorganization, hydration level, and confinement within surfactant and polymer assemblies. Here, we investigate the interaction of a cationic polyelectrolyte, poly(diallyl dimethylammonium chloride) (PDADMAC), with micelles of differently charged surfactants using 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) as an ESPT probe. We used three surfactants: anionic sodium dodecyl sulfate (SDS), cationic dodecyl trimethylammonium bromide (DTAB), and zwitterionic N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB12), possessing the same alkyl (dodecyl) chain but varying headgroup charges. The fluorescence of HPTS residing initially within the micellar medium modulates differently in the presence of PDADMAC. For the anionic SDS and cationic DTAB micelles, the emission spectrum of HPTS does not alter significantly; however, for SB12 micelles, the emission spectrum undergoes a strong modulation upon adding the polyelectrolyte. The emission intensities quench strongly at a low concentration of PDADMAC but recover at a higher concentration. The emission intensity ratio of the two emission bands also changes significantly, implying strong modulation of the ESPT process with varying PDADMAC concentrations. The time-resolved area normalized emission spectra (TRANES) disclose single isoemissive points in the SB12 micelle at low and high concentrations of PDADMAC but two different isoemissive points (one characteristic of the SB12 micelle at 500 nm and another characteristic of the PDADMAC interface at 480 nm) in the mixed assembly at an intermediate concentration. Detailed analysis suggests that the polyelectrolyte can enforce the transfer of the anionic probe HPTS from the zwitterionic micelle to the PDADMAC assembly above a specific PDADMAC concentration. The transfer of the molecular probe between two assemblies resembles a drug sequestration event, and the study reveals necessary emission signatures.

Exploration of Self-Aggregation of Coumarin 7 and Coumarin 30 in Water: Role of β-Cyclodextrin as a Modulator.

Steady-state and time-resolved spectroscopic studies demonstrate that two members of the coumarin class of dyes, coumarin 7 (C7) and coumarin 30 (C30), undergo self-aggregation in water. The development of hypsochromically shifted new absorption bands in addition to the existing monomer bands with an increase in concentration of the dyes in an aqueous medium suggests that the aggregates are of H-type. An absorption-based kinetic study reveals that the rate of aggregation of C30 is an order of magnitude faster than that of C7. Second-order rate kinetics, as obtained from the half-life (t1/2) data, implies that the aggregates are dimeric in nature. Observations of isosbestic points in area-normalized absorption spectra (ANAS) and isoemissive points in area-normalized fluorescence excitation spectra (ANFES) and time-resolved area-normalized emission spectra (TRANES) establish that ground-state monomer ⇌ dimer equilibria for both of the systems are preserved in the photoexcited state. The present study further establishes that β-cyclodextrin is the most efficient of the three common cyclodextrins in shifting the equilibria toward the monomer by encapsulating the monomers within its cavity, making β-CD a convenient modulator to control the self-aggregation process. Dynamic light scattering (DLS), quantum chemical calculations, and molecular docking studies provide further support to our propositions.

Isoemissive Photoluminescence from a Quaternary System of Valley-Polarized, Defect-Bound Excitons and Trions in Two-Dimensional Transition Metal Dichalcogenides

Interest in the use of local band extrema, known as valleys, as a possible information carrier has in recent years been advanced by the isolation of 2D transition metal dichalcogenides (TMDCs). In some monolayer TMDCs, the structural inversion asymmetry leads to spin-valley locked states that present potential advantages for information storage and manipulation beyond existing charge- and spin-based semiconductor technologies. However, understanding of the role of defects on exciton recombination and valley scattering remains lacking and limits progress toward applications. Here, we report gate voltage-dependent polarized photoluminescence measurements on CVD-grown monolayer WS2 and MoS2 field-effect transistors and observe a quaternary system of valley polarized excitons and trions. Intriguingly, we find the spectra to exhibit isoemissive points, which were previously seen only as evidence of a binary system. We propose a rate model to explain the existence of isoemission in a quaternary system and find that the presence of isoemissive points imply a kinetic relationship between recombination and valley scattering. This relationship sheds light on the inverse relation seen between the polarization degree and photoluminescence intensity. Further consideration of the phenomenon also led to the identification of a possible new mechanism for charge carrier detrapping using the energy released from trion recombination.

Concurrent ground and excited state proton transfer of (E)-2-((napthalen-2-ylimino)-methyl)phenol: Modulation in micellar media

The phenomena of ground and excited state proton transfer (GSPT and ESPT) of the synthesized Schiff base, (E)-2-((napthalen-2-ylimino)-methyl)phenol (NMP) have been unraveled in aqueous medium using various spectroscopic techniques. Changes in the absorption and emission spectra of an aqueous solution of the probe upon addition of an external base are ascribed to the proton transfer in both the ground and the excited states. Existence of equilibria between the neutral and the deprotonated species in the two electronic states is established from the observation of the isosbestic and isoemissive points in the absorption and emission spectra. Acid dissociation constants in the ground state (pKa) and photoexcited state (pKa*) of NMP in aqueous medium have been derived from the steady-state experiments using Weller’s method. The time resolved experiments clearly indicate the reversibility of the proton transfer reaction in the photo-excited state. The pKa* value determined from the kinetic measurements corroborates that obtained from the steady state methods. The higher value of pKa* than pKa is rationalized from the higher electron density on oxygen in the photoexcited state relative to the ground state, as obtained from the Mulliken charge densities through quantum chemical calculations. Extending our studies to the micellar environments, it is established that the deprotonation process is favored in cationic micelles while the same is disfavored in anionic or non-ionic micelles. This has been rationalized in terms of the surface charge characteristics of the individual micelles.

Enhanced Excited-State Proton Transfer via a Mixed Water-Methanol Molecular Bridge of 1-Naphthol-5-Sulfonate in Methanol-Water Mixtures.

We used steady-state and time-resolved fluorescence techniques to study the excited-state proton transfer (ESPT) and the nonradiative properties of two irreversible photoacids, 1-naphthol-4-sulfonate (1N4S) and 1-naphthol-5-sulfonate (1N5S). We found that the ESPT rate constant of 1N4S in water is 2.2 × 1010 s-1, whereas in methanol, it is smaller by about 3 orders of magnitude and is not observed. The ESPT process of 1N5S competes with a major nonradiative process of equal rate and kPT of 2.2 × 1010 s-1. In methanol-water mixtures of χH2O = 0.2, the fluorescence lifetime of the ROH form of 1N5S is lower by a factor of 10 than that in pure methanol. In the steady-state fluorescence spectra of 1N5S in methanol-water mixtures, there are two iso-emissive points, one for χH2O < 0.2 and one for χH2O > 0.3. This large reduction in fluorescence intensity and the two iso-emissive points are explained by the existence of a mixed water-methanol bridge of about three molecules that connects the proton donor 1-OH with the 5-sulfonate in mixtures of χH2O < 0.2. The bridge enhances both the ESPT and the nonradiative processes. For 1N4S in methanol-water mixtures at χH2O ≈0.2, the reduction in the fluorescence lifetime is only by ∼30%, and only one iso-emissive point exists in the steady-state fluorescence spectra for 0 <χH2O < 1. TD-DFT computations show that a mixed bridge of one water molecule and two methanol molecules that connects the 1-OH with 5-sulfonate is more stable by 7.7 kcal/mol than the 1-OH reactant in the S1 state, and the barrier is only 8.0 kcal/mol. The nonradiative channel is because the S2 dark state is about 4.6 kcal/mol higher than the S1 state.

Physicochemical study of the interaction of lysozyme with surface active ionic liquid 1-butyl-3-methylimidazolium octylsulfate [BMIM] [OS] in aqueous and buffer media

Abstract The interactions of the surface active ionic liquid (IL), 1-butyl-3-methylimidazolium octylsulfate [BMIM] [OS], with lysozyme were studied using tensiometry, turbidimetry, absorbance, fluorescence spectra, circular dichroism (CD), and Fourier transformed infra red (FTIR) techniques in phosphate buffer medium (pH 6 and 8) at 298 K. In the studied media coacervate was formed and the coacervation process depended on protein concentration. Isoemissive points were observed in emission spectra of lysozyme with IL. The isoemissive wavelength depended on the pH of the solution and also on [lysozyme]. The denaturation of lysozyme depended on the medium pH and the added [IL] as revealed from the CD and FTIR measurements. Molecular docking has provided information of the region of the lysozyme surface where the IL started to bind with it.

Silica nanoparticles with covalently attached fluorophore as selective analyte-responsive supramolecular chemoreceptors

Silica core-shell nanoparticles with SiO2 cores and an alkoxysilyl derivative of dibenzoylmethanatoboron difluoride (A-DBMBF2) fluorophore covalently attached to the core surface have been synthesized. It is shown that these nanoparticles can be used as the basis of selective sensor materials capable of detecting benzene, toluene, and xylene vapors. As benzene or its methyl derivatives are adsorbed on the surface of nanoparticles, a quenching of A-DBMBF2 fluorescence occurs while an A-DBMBF2/analyte exciplex fluorescence buildup emerges. The position of isoemissive points in the fluorescence spectra is specific for each analyte and can be used for its identification.

Design and synthesis of luminescence chemosensors based on alkynyl phosphine gold(i)–copper(i) aggregates

Receptor-containing polynuclear mixed-metal complexes of gold(I)-copper(I) 1-3 based on a [{Au(3)Cu(2)(C≡CPh)(6)}Au(3){PPh(2)-C(6)H(4)-PPh(2)}(3)](2+) (Au(6)Cu(2)) core with benzo-15-crown-5, oligoether and urea binding sites were designed and synthesized, respectively. These complexes exhibited remarkably strong red emission at ca. 619-630 nm in dichloromethane solution at room temperature upon photoexcitation at λ > 400 nm, with the emission quantum yield in the range 0.59-0.85. The cation-binding properties of 1 and 2 and the anion-binding properties of 3 were studied using UV-vis, emission and (1)H NMR techniques. Complex 1, with six benzo-15-crown-5 pendants, was found to show a higher binding preference for K(+), with a selectivity trend of K(+)≫ Cs(+) > Na(+) > Li(+). The addition of metal ions (Li(+), Na(+), K(+) and Cs(+)) to complex 1 led to a modest emission enhancement with a concomitant slight blue shift in energy and well-defined isoemissive points, which is attributed to the rigidity of the structure and the inhibited PET (photo-induced electron transfer) process from the oxygen to the aggregate as a result of the binding of the metal ion. The six urea receptor groups on complex 3 were found to form multiple hydrogen bonding interactions with anions, with the positive charge providing additional electrostatic interaction for anion-binding. The anion selectivity of 3 follows the trend F(-) > Cl(-)≈ H(2)PO(4)(-) > Br(-) and the highest affinity towards F(-) is attributed to the stronger basicity of F(-), as well as its good size match with the cavity of the urea pocket.

Fluorescence Spectroscopic Evidence for Hydrogen Bonding and Deprotonation Equilibrium between Fluoride and a Thiourea Derivative

Interaction of anions with thiourea-linked acridinedione fluorophore was studied by absorption, (1)H NMR, steady-state and time-resolved fluorescence techniques. Addition of AcO(-) and H(2)PO(4)(-) shows a genuine H-bonded complex with thiourea receptor; whereas, F(-) shows stepwise H-bonding and deprotonation of thiourea NH as confirmed by (1)H NMR titration. Free receptor 1 shows emission maximum at 418 nm; whereas, H-bonded complex of 1·F(-) shows a new redshifted emission maximum at 473 nm and the deprotonated 1 exhibits an emission peak at 502 nm. Presence of these three different emitting species was probed by 3D emission spectroscopic studies. Equilibrium between the free receptor 1, 1·F(-) H-bonded complex and deprotonated 1 was confirmed by time-resolved fluorescence studies. Time-resolved area normalised emission spectra (TRANES) of 1 in the presence of F(-) shows two isoemissive points at 456 and 479 nm between time delays of 0-0.5 ns and 1-20 ns, respectively, due to the existence of three emitting species in equilibrium. Observation of such an equilibrium based on fluorescence spectroscopic studies further proves the earlier reported absorption and (1)H NMR spectroscopic studies of H-bonding and deprotonation processes and also illustrates the dynamics of anion-receptor interactions.

Intramolecular Charge Transfer with 4-Fluorofluorazene and the Flexible 4-Fluoro-N-phenylpyrrole

With 4-fluorofluorazene (FPP4F) and its flexible counterpart 4-fluoro-N-phenylpyrrole (PP4F) an intramolecular charge transfer (ICT) reaction occurs in the singlet excited state in sufficiently polar solvents. The ICT reaction begins to appear in tetrahydrofuran (epsilon = 7.4) for FPP4F and in the more polar 1,2-dichloroethane (epsilon = 10.4) with PP4F, showing its presence by dual fluorescence from a locally excited (LE) and an ICT state. Only LE fluorescence is observed in less polar solvents such as n-hexane. The ICT reaction is more pronounced with FPP4F than for PP4F, due to the smaller energy gap DeltaE(S1,S2) of the former molecule, in accordance with the PICT model. The occurrence of an ICT reaction is confirmed by the ICT dipole moments mu(e)(ICT) of 12 D (FPP4F) and 10 D (PP4F), clearly larger than mu(e)(LE) of approximately 4 D for FPP4F and PP4F. Isoemissive points are found in the fluorescence spectra of FPP4F and PP4F in acetonitrile (MeCN), ethyl cyanide (EtCN), and n-propyl cyanide (PrCN) as a function of temperature, confirming the two-state (LE and ICT) reaction mechanism. From plots of the logarithm of the ICT/LE fluorescence quantum yield ratio versus the reciprocal absolute temperature in these solvents, the ICT reaction enthalpies DeltaH are determined, with larger -DeltaH values for FPP4F than for PP4F: 19.2 as compared with 14.9 kJ/mol in MeCN, as an example. The picosecond fluorescence decay of PP4F at -45 degrees C becomes slower with decreasing solvent polarity, 5.1 ps (MeCN), 14 ps (EtCN), and 35 ps (PrCN), from which the LE --> ICT reaction rate constant is calculated, decreasing from 19 x 10(10) to 2.1 x 10(10) s(-1) between MeCN and PrCN. The femtosecond LE excited-state absorption spectra of FPP4F and PP4F do not undergo any time development in n-hexane (no ICT reaction), but show a fast ICT reaction in MeCN at 22 degrees C, with decay times of 1.1 ps (FPP4F) and 3.3 ps (PP4F). It is concluded that FPP4F and PP4F have a planar ICT state (PICT model), indicating that a perpendicular twist of the donor and acceptor subgroups in a donor/acceptor molecule is not a requirement for fast and efficient ICT. The molecular structures of FPP4F and PP4F obtained from X-ray crystal analysis reveal that the pyrrole group of PP4F is twisted over an angle theta = 25 degrees relative to the fluorophenyl moiety in the ground state, whereas as expected FPP4F is practically planar (theta = 2 degrees). The pyrrole-phenyl bond length of FPP4F (140.7 pm) is shorter than that for PP4F (141.8 pm).

DNA Duplex-Based Fluorescence Probes/Sensors Using Monomer-Excimer Switching

In this review, we report DNA duplex-based fluorescence probes/sensors using pyrene monomer-excimer switching. The review mainly comprises two topics: 1) excimer-monomer switching molecular beacons (EMS-MB) and 2) monomer-excimer switching sensors based on the structural motif of antibodies. The EMS-MBs have two pyrene fluorophores connected both at 3' and 5' ends of a single-stranded oligonucleotide. Emission switching occurs from excimer to monomer accompanying isoemissive points when the probes hybridized with target DNAs. The isoemissive points indicate the presence of only two fluorescent species, nonhybridized and hybridized probes in the mixtures, and thereby unambiguous detection of the targets is available. The probes can detect target 19-mer DNAs and can discriminate the targets from their single-nucleotide mismatches at 1 nM concentration. Furthermore, the EMS-MBs have been recently applied to kinetic study for RNase H activity by Tan et al. The structures and emission-switching properties of the EMS-MBs encourage us to develop a new class of fluorescent sensors based on the structural motif of antibodies. The sensors consist of three functional regions, benzo-15-crown-5 ether (or per-O-methylated beta-cyclodextrin), DNA, and pyrene as guest-binding, dimerizing, and sensing sites, respectively. The crown- and CD-modified sensors can detect potassium cation and porphyrin derivatives, respectively, by monomer-excimer emission switching.

New Insights on the Photophysical Behavior of PRODAN in Anionic and Cationic Reverse Micelles: From Which State or States Does It Emit?

6-propionyl-2-(N,N-dimethyl)aminonaphtahalene, PRODAN, is widely used as a fluorescent molecular probe because of its significant Stokes shift in polar solvents. It is an aromatic compound with intramolecular charge-transfer states (ICT) that can be particularly useful as a sensor. The nature of the emissive states has not yet been established despite the detailed experimental and theoretical investigations done on this fluorophore. In this work, we performed absorption, steady-state, time-resolved fluorescence (TRES) and time-resolved area normalized emission (TRANES) spectroscopies on the molecular probe PRODAN in the anionic water/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/n-heptane and the cationic water/benzyl-n-hexadecyl dimethylammonium chloride (BHDC)/benzene reverse micelles (RMs). The experiments were done by varying the surfactant concentrations at a fixed molar ratio (W = [H2O]/[Surfactant]) and changing the water content at a constant surfactant concentration. The results obtained varying the surfactant concentration at W = 0 show a bathochromic shift and an increase in the intensity of the PRODAN emission band due to the PRODAN partition process between the external solvent and the RMs interface. The partition constants, Kp, are quantified from the changes in the PRODAN emission spectra and the steady-state anisotropy (<r>) with the surfactant concentration in both RMs. The Kp value is larger in the BHDC than the AOT RMs, probably due to the interaction between the cationic polar head of the surfactant and the aromatic ring of PRODAN. The partition process is confirmed with the TRES experiments, where the data fit to a continuous model, and with the time-resolved area normalized emission spectroscopy (TRANES) spectra, where only one isoemissive point is detected. On the other hand, the emission spectra at W = 10 and 20 show a dual fluorescence with a new band that emerges in the low-energy region of the spectra, a band that was previously assigned to the PRODAN emission from the water pool of RMs. Our studies demonstrate that this band is due to the emission from an ICT state of the molecular probe PRODAN located at the interface of the RMs. These results are also confirmed by the lifetime measurements, the TRES experiments where the results fit to a two-state model, and the time-resolved area normalized emission spectroscopy (TRANES) spectra where three or two isoemissive points are detected in the AOT and BHDC RMs, respectively. In the AOT RMs, Kp values obtained at W = 10 and 20 are almost independent of the water content; the values are higher for the BHDC RMs due to the higher micropolarity of this interface.

Trinuclear Gold(I) Triazolates: A New Class of Wide-Band Phosphors and Sensors

A new cyclic gold(I) triazolate trimer, [Au(3,5-i-Pr2Tz)]3 (1), exhibits fully overlapping aurophilically bonded dimer-of-trimer units that lead to multiple phosphorescence bands in both the solid state and solution. The conformation of the hexanuclear unit exhibits reversible interconversion between C2 and D3 effective symmetries, depending on the crystal temperature or solution concentration, the variation of which leads to isoemissive and isosbestic points. Solutions of 1 exhibit remarkable quenching properties that demonstrate molecular recognition with high selectivity and hypersensitivity for some reagents, as influenced by protonation via Brønsted acids, pi intercalation, and/or energy transfer. The quenched phosphorescence of 1 by acetic acids can be regenerated by NEt3.

Conformational equilibria in EE-1,3-di-(3′-thienylethenyl)benzene: One-way adiabatic interconversion of rotamers in S1 and their excited state properties

A refined procedure of the kinetic fluorescence analysis (KFA) (accounting for the lack of isoemissive points in the fluorescence spectra) has been applied to study the rotamerism of the EE isomer of 1,3-di-(3′-thienylethenyl)benzene [(3TE)2B]. The effect of the excitation wavelength and temperature on the fluorescence spectra, quantum yields and decay profiles allowed the excited state properties (radiative and reactive) of the two most abundant conformers (rotamers), implying the simultaneous, restricted rotation around the quasi-single bonds of the ethenic bridges with both the central ring and the 3′-thienyl groups, to be obtained. At high temperature (above 320K), another activated process of the lowest excited singlet state, implying the interconversion of the short-lived rotamer into the longer-lived one, is operative. The EE→EZ photoisomerization was also investigated as a function of temperature.The present results lead to a complete and quantitative description of the conformational equilibrium including the radiative and radiationless relaxation rate parameters of the lowest excited states of the two rotamers. A simultaneous singlet and triplet mechanism for the geometrical photoisomerization, with an involvement of the former above room temperature only, seems to reasonably account for the experimental results. The role played by the rotamers in the photoreactive behaviour of (3TE)2B is also discussed.

Shape and Position of 4-Aminophthalimide (4-AP) Time-Resolved Emission Spectra (TRES) versus Sodium Dodecyl Sulfate SDS Concentration in Micellar Solutions: The Partitioning of 4-AP in the Micellar Phase and in Water Surrounding the Micelles

Time-resolved emission spectra (TRES) of 4-aminophthalimide (4-AP) dissolved in water solutions of sodium dodecyl sulfate (SDS) for three surfactant concentrations (0.05, 0.15, and 0.45 M) have been determined. The fraction of 4-AP dissolved in the water phase surrounding the micelles has been shown to increase with decreasing concentration of the surfactant. To obtain TRES of 4-AP present exclusively in micelles, a method of subtraction of the contribution of the emission originating from 4-AP present in the water phase surrounding the micelles from the total emission of the probe dissolved in SDS solution has been proposed. The consequences of failing to take into account the partitioning of 4-AP between the water and micellar phases are illustrated by some exemplary TRES results, taken before and after the subtraction of the emission originating from 4-AP present in the water phase. Together with the time of appearance and presence of isoemissive points in the time-resolved area-normalized emission spectra (TRANES), these results have shown a clear dependence of the rate and character of the 4-AP TRES changes on the SDS concentration. In connection with our earlier results and literature data, it has been concluded that the concentration of the water solubilized in micelles is the main factor determining the rate and character of these changes.

Unambiguous detection of target DNAs by excimer-monomer switching molecular beacons.

A new class of molecular beacons were developed in which pyrene fluorophores were connected both at 3' and 5' ends of a single-stranded oligonucleotide. The two pyrene-based fluorophores were synthesized from the same starting material, so that the preparation of the beacons was simplified. The detection strategy of the beacons for target DNAs is based on "excimer-monomer emission switching" of the pyrene fluorophores: excimer emission of the pyrene moieties changed to monomer one when the beacons hybridized with the targets. This type of two-state mode of fluorescence allows unambiguous detection of the target DNAs because strict 1:1 correlation between the nonhybridized and the hybridized beacons can be monitored by the presence of isoemissive points of the fluorescence changes. The beacons can detect target 19-mer DNAs and can discriminate the targets from their single-nucleotide mismatches at 1 nM concentration. Advantages of the excimer-monomer switching molecular beacons were discussed in comparison with conventional ones.

Application of time resolved area normalized emission spectroscopy to multicomponent systems

Time resolved emission spectroscopy (TRES) provides information on the excited state kinetics and heterogeneity of emissive species in a system. Time resolved area normalized emission spectroscopy (TRANES), an extension to TRES, is a novel, model-free method for the analysis of intrinsic or extrinsic fluorescence probes in complex chemical and biophysical systems [Koti, Krishna, and Periasamy, J. Phys. Chem. A 105, 1767 (2001)]. Observation of a single isoemissive point in TRANES analysis of fluorescence is an unambiguous indication of the presence of two emissive species in the system. The presence of multiple isoemissive points in TRANES spectra is confirmed using simulation and experimental data of multicomponent systems.

Absorption, fluorescence and transient triplet–triplet absorption spectra of zinc tetramethylpyridylporphyrin in reverse micelles and microemulsions of aerosol OT–(AOT)

The photophysical properties of water soluble Zn(II) meso-tetrakis(N-methylpyridinium-4-yl)-porphyrin were studied in reverse micelles of the iso-octane–AOT–water system. Broadening of absorption and emission bands as well as shifts and increase of maxima intensities were observed with different values of W0 = [H2O]/[AOT], with a fixed AOT (0.1 M) concentration. These effects are generally good evidence for distinct probe locations inside the aggregates, but no clear evidence of equilibria (isosbestic and isoemissive points) was detected. The decomposition of the Soret absorption band into Gaussian functions showed the presence of two species, one near the well known blue Soret band in water (435 nm) and another more displaced to the red (442 nm). Transient triplet–triplet absorption spectra confirm the different locations of the porphyrin in the micelle interface. One of the species, which is located in a more hydrophobic region of the interface has a triplet decay constant around ten times slower than the other one located at the hydrophilic region of the interface with lower singlet energy and a fast triplet decay rate constant.

Rotamerism and trans–cis photoisomerization of 1-(2-naphthyl)-2-(n′-pyridyl)ethylenes studied by stationary and pulsed fluorescence techniques

A study of the fluorescence properties (spectra, quantum yields, decay curves) of trans-1-(2-naphthyl)-2-(n′-pyridyl)ethylenes as a function of the excitation wavelength and temperature has allowed the observation of a dynamic equilibrium between different conformers. The fluorimetric analysis put forward previously for a mixture of rotamers with isoemissive points has now been extended to systems without isoemissive points and successfully applied to derive the distinct kinetic parameters of 1-(2-naphthyl)-2-(3′-pyridyl)ethylene. For the ortho and paran-positional isomers, only the separate lifetimes and the enthalpy difference between the two rotamers have been evaluated. The properties of the systems studied have been compared to those of related hydrocarbons and discussed from the point of view of their implication in the trans–cis photoisomerization mechanism.

Temperature effects on the relaxation parameters of the 1-methylnaphthalene excimer

The excimer lifetime τ D and the excimer fluorescence efficiency for 1-methylnaphthalene in ethanol have been determined between −30 and 60°C. Expressing the rate constant for excimer deactivation. k D, in terms of radiative k FD) and nonradiative ( k ID) processes as k D = k FD + k ID it is found that k FD is independent of tempeature and k ID = 5 × 10 6 + 3.2 × 10 11 exp (-Δ E/RT) sec −1, where Δ E = 6.7 kcal mole −1. The behaviour of k FD and k ID with temperature and the appearance of isoemissive points in a limited region of temperature are discussed.