F. C. De Schryver

Terrylenimides: New NIR Fluorescent Dyes

Terrylenimides 3 and 4 represent a new class of blue colorants, exhibiting absorption maxima at 650 to 700 nm and fluorescence emissions in the NIR region (673 to 750 nm). The terrylenimides were synthesized by means of various organometallic coupling reactions, catalyzed by transition metal complexes (Ni(o) , Pd(o) ) and starting from the aromatic bromides, boronic acids, or organotin compounds. The terrylenimides have all the properties expected of excellent fluorescent dyes: high extinction coefficients, high fluorescence quantum yields, and very good thermal, chemical, and photochemical stabilities. Owing to its extended π system, 3 can reversibly accept four negative charges. By varying the substituents, 3 and 4 can be modified to serve either as soluble dyes or as insoluble pigments.

Possibilities and limitations of the time-correlated single photon counting technique: a comparative study of correction methods for the wavelength dependence of the instrument response function

Abstract One of the principal reasons for poor fits in time-correlated single photon counting fluorescence decay measurements is the wavelength dependence of the instrument response function. Over the years several correction methods have been developed to account for or correct for this effect. The most recent and widely used procedures are critically compared in this paper. A comparison of the channel shift technique, the pseudoscatterer technique, the excitation pulse-shape mimic technique and the delta function convolution method demonstrates that the latter method is the superior one. This method requires the measurement of the fluorescence decay of a reference compound with single exponential decay kinetics under identical conditions as used for the sample. A modified functional form is used to describe the sample decay law. Since no experimental data are altered in any way, a correct residual analysis is possible. Simulations show that fluorescence lifetimes can be recovered accurately, as long as the reference decay time is sufficiently different from that of the sample. Biexponential decays can be resolved successfully when the two decay times are well separated and different from that of the reference. The difference between the standard normal variates of chi-square for single and double exponential fits can be used to distinguish between mono- and bi-exponential decays. The usefulness of the delta function convolution method is demonstrated by fluorescence decay measurements of a series of different samples with very short decay times. Fluorescence lifetimes as short as 10 ps could be resolved accurately and reliably.

Conformational rearrangements in and twisting of a single molecule

Single molecule spectroscopy is used to obtain detailed information on the photophysical properties of immobilized perylenediimide-based molecules, substituted in the bay positions. The fluorescence spectra recorded for numerous single molecules show a clear bimodal distribution of the peak position. Within the low energy component of the distribution, two different vibronic shapes of the emission spectrum can be seen, which can be correlated to different decay times. We show that former observation can be explained by conformational changes of the bay substituents while the latter are related to twisting of the single molecule around the central perylenediimide long axis.

Quenching of 1-methylpyrene by Cu2+ in sodium dodecylsulfate. A more general kinetic model

Abstract Experimental evidence of cation exchange between anionic micelles is presented. The process is revealed by a kinetic study of fluorescence quenching of solubilized aromatic molecules. This phenomenon is accounted for in a kinetic scheme more general than the one formerly proposed by other authors.

PHOTOPHYSICS OF THE FLUORESCENT K+ INDICATOR PBFI

The fluorescent indicator PBFI is widely used for the determination of intracellular concentrations of K+. To investigate the binding reaction of K+ to PBFI in the ground and excited states, steady-state and time-resolved measurements were performed. The fluorescence decay surface was analyzed with global compartmental analysis yielding the following values for the rate constants at room temperature in aqueous solution at pH 7.2: k01 = 1.1 x 10(9) s-1, k21 = 2.7 x 10(8) M-1s-1, k02 = 1.8 x 10(9) s-1, and k12 = 1.4 x 10(9) s-1. k01 and k02 denote the respective deactivation rate constants of the K+ free and bound forms of PBFI in the excited state. k21 represents the second-order rate constant of binding of K+ to the indicator in the excited state whereas k12 is the first-order rate constant of dissociation of the excited K(+)-PBFI complex. From the estimated values of k12 and k21, the dissociation constant Kd* in the excited state was calculated. It was found that pKd* (-0.7) is smaller than pKd (2.2). The effect of the excited-state reaction can be neglected in the determination of Kd and/or the K+ concentration. Therefore, intracellular K+ concentrations can be accurately determined from fluorimetric measurements by using PBFI as K+ indicator.

Expression of chirality by achiral coadsorbed molecules in chiral monolayers observed by STM

The supramolecular packing mode of physisorbed monolayers built up by chiral isophthalic acid derivatives and coadsorbed achiral solvent molecules was imaged at the liquid/graphite interface with scanning tunneling microscopy (STM). The picture on the right shows the submolecularly resolved STM image of an enantiomorphous domain composed of the R enantiomer of the isophthalic acid derivative studied and 1-heptanol molecules; the latter express the chirality of the monolayer. Upon adsorption a racemic mixture is separated into enantiomorphous domains.

Fluorescence quenching in micelles: A theoretical model for the intramicellar first order quenching rate constant

Considering the fact that in a micellar medium solubilized molecules are mostly present near the surface, a solution of Fick’s diffusion law on a spherical surface is obtained to calculate the rate of diffusion controlled reactions in a micelle. It is shown that transients are small. Corrections are applied for the radial distribution of probe and quencher. The model is extended to reactions slower than diffusion controlled.

Photophysical study of a multi-chromophoric dendrimer by time-resolved fluorescence and femtosecond transient absorption spectroscopy

Abstract The photophysical properties of a dendrimer ( 1 ) bearing eight peryleneimide chromophores on a polyphenylene core were investigated by picosecond fluorescence and femtosecond transient absorption techniques. A peryleneimide attached to a hexaphenylbenzene unit ( 2 ) served as model compound. Multi-exponential fluorescence and fs-transient absorption decays were observed only for 1 , which could therefore be attributed to excimer-like interactions among neighbouring peryleneimides. The comparative time-resolved polarisation results on 1 and 2 indicate the occurrence of different depolarisation processes in the dendrimer assignable to the rotation of the molecule, a fractional motion maybe of a dendrimer branch and to an intramolecular energy transfer process.

PHOTOPHYSICAL PROPERTIES OF 2‐NITRO‐5,10,15,20‐TETRA‐p‐TOLYLPORPHYRINS

Abstract— Tetraarylporphyrins substituted with nitro groups at fipyrrolic positions are potential candidates for electron‐accepting pigments in model systems for photosynthesis. The photophysics of 2‐nitro‐5,10,15,20‐tetra‐p‐tolylporphyrin and its zinc analog have been studied in order to evaluate this potential. The ground state absorption spectrum, the triplet‐triplet absorption spectrum, the fluorescence emission spectrum, and associated photophysical parameters have been determined. The molecules have short singlet lifetimes and anomalous temperature‐ and solvent‐dependent emission spectra which are consistent with the formation of an intramolecular charge transfer state of the type P+ ‐NO2; in which the nitro group is twisted about its bond to the porphyrin, relative to the ground state conformation.

Organic electronics: Supra solutions

Supramolecular interactions between organic molecules not only enable them to self-organize into large regular assemblies, but also enhance their electronic and luminescent properties, which should help to improve the performance of organic devices.