Andrzej M. Turek

Quenching of exciplex fluorescence by lithium perchlorate in acetonitrile

Abstract Fluorescence quenching of exciplexes, formed by diffusion from excited electron acceptors such as 9-cyanoanthracene and 9,10-dicyanoanthracene, and electron donors such as biphenyl, durene and naphthalene, by lithium perchlorate, in acetonitrile has been measured using fluorescence techniques. It has been found that lithium perchlorate is an efficient quencher of bimolecular charge transfer forms such as exciplexes. The quenching efficiency depends strongly on the charge transfer contribution in the overall exciplex. The Stern-Volmer plots constructed from the exciplex lifetimes or exciplex quantum yields possess negative curvatures. This may be explained on the basis of the partial dissociation of lithium perchlorate in acetonitrile.

Photoisomerization of cis,cis-1,4-diphenyl-1,3-butadiene in glassy media at 77 K: the bicycle-pedal mechanism

The cis-trans photoisomerization of cis,cis-1,4-diphenyl-1,3-butadiene in a soft isopentane glass at 77 K gives significant two-bond photoisomerization in contrast to solution and hard glassy media where only one-bond photoisomerization takes place.

Influence of temperature on fluorescence quenching of 9,10-dicyanoanthracene by durene in acetonitrile

Abstract The effect of temperature on the fluorescence quenching of 9,10-dicyanoanthracene (DCA) by durene in acetonitrile has been investigated. The fluorescence quenching in the system is accompanied by a weak exciplex emission. The exciplex fluorescence quantum yields and exciplex lifetimes depend strongly on temperature and the exciplex emission is efficiently quenched by lithium perchlorate. The most important pathway of deactivation of the contact ionic pair (CRIP) is solvation of the CRIP. The structure of the CRIP has been discussed on the basis of the analysis of the exciplex fluorescence spectra as well as the temperature dependence of the fluorescence quenching rate constants.

The Photochemistry of a bis-Crown Ether Based on Benzobis(thiazole) and Its Alkaline Earth Metal Cation Complexes

Abstract Irradiation of acetonitrile solutions of the bis-crown ether E,E-2,7-bis[2-(6,7,9,10,12,13,15,16-octahydro-5,8,11,14,17-pentaoxabenzocyclopentadecen-2-yl)vinyl]-benzo[1,2-d;3,4-d′]bisthiazole (hereafter, 1) gives efficient E → Z photoisomerization (initial φtrans→cis = 0.48), leading to λexc-dependent quasi-photostationary states composed primarily of E,Z and E,E isomer mixtures. Further irradiation gives [2 + 2]-cycloadducts of 1. In the presence of Ba2+ ions, essentially quantitative formation of 2 : 2 complexes, 12·(Ba2+)2 controls the photochemical outcome. E → Z photoisomerization of the ligand is entirely suppressed and efficient intramolecular [2 + 2]-photocycloaddition in the complexes leads to cyclobutane dimers of 1 (φCB = 0.26). The reactivity of 1 in the presence of Mg2+ ions for which 1 : 2 complex formation dominates gives both cis–trans photoisomerization and enhanced photocycloaddition.

Site induced structures in the lowest electronic transitions of naphthalene-H8 and -D8 in rare-gas matrices

Abstract Phosphorescence spectra and S1 ← S0 excitation spectra of naphthalene-h8 and -d8 in Ar, Kr and Xe matrices have been investigated with medium spectral resolution. Observed site structures in the spectra are depemdent on deposition conditions and annealing. Comparison of calculated and experimentally observed site shifts suggests that the matrix structure is preferentially ccp. Possible assignments of the site structures are given. Geometrical changes of naphthalene in excited states are important factors determining site structure in electronic transitions.

Luminescence of naphthalene in an argon matrix. Geometry changes and multiplet structures

Abstract The stabilization energies of naphthalene in the ground and excited states for various substitutional sites in an argon matrix have been calculated. The major multiplet splittings in the naphthalene phosphorescence depend on the size of the sites The geometry change between the ground and excited states of naphthalene was found to be the important factor which modifies the energies of the multiplet components in S 1 → S o and T 1 → S o transitions.

Modelling of site induced structures in the electronic transitions of naphthalene in rare gas matrices

Various insertion modes of the naphthalene molecule in the rare gases were analysed by constructing the intermolecular potential energy functions for naphthalene in the substitutional sites. Spectral shifts. Stokes shifts and local librational frequencies for naphthalene-d8 in the sites have been evaluated. The parametrization of the interaction potential for the excited naphthalene in rare gases and the effects of geometry changes and rigidity of matrix are discussed.

Fast Decomposition of Three-Component Spectra of Fluorescence Quenching by White and Grey Methods of Data Modeling

Abstract‘White’ and ‘grey’ methods of data modeling have been employed to resolve the heterogeneous fluorescence from a fluorophore mixture of 9-cyanoanthracene (CNA), 10-chloro-9-cyanoanthracene (ClCNA) and 9,10-dicyanoanthracene (DCNA) into component individual fluorescence spectra. The three-component spectra of fluorescence quenching in methanol were recorded for increasing amounts of lithium bromide used as a quencher. The associated intensity decay profiles of differentially quenched fluorescence of single components were modeled on the basis of a linear Stern-Volmer plot. These profiles are necessary to initiate the fitting procedure in both ‘white’ and ‘grey’ modeling of the original data matrices. ‘White’ methods of data modeling, called also ‘hard’ methods, are based on chemical/physical laws expressed in terms of some well-known or generally accepted mathematical equations. The parameters of these models are not known and they are estimated by least squares curve fitting. ‘Grey’ approaches to data modeling, also known as hard-soft modeling techniques, make use of both hard-model and soft-model parts. In practice, the difference between ‘white’ and ‘grey’ methods lies in the way in which the ‘crude’ fluorescence intensity decays of the mixture components are estimated. In the former case they are given in a functional form while in the latter as digitized curves which, in general, can only be obtained by using dedicated techniques of factor analysis. In the paper, the initial values of the Stern-Volmer constants of pure components were evaluated by both ‘point-by-point’ and ‘matrix’ versions of the method making use of the concept of wavelength dependent intensity fractions as well as by the rank annihilation factor analysis applied to the data matrices of the difference fluorescence spectra constructed in two ways: from the spectra recorded for a few excitation lines at the same concentration of a fluorescence quencher or classically from a series of the spectra measured for one selected excitation line but for increasing concentration of the quencher. The results of multiple curve resolution obtained by all types of the applied methods have been scrutinized and compared. In addition, the effect of inadequacy of sample preparation and increasing instrumental noise on the shape of the resolved spectral profiles has been studied on several datasets mimicking the measured data matrices. Graphical Abstractᅟ

π–π-Induced aggregation and single-crystal fluorescence anisotropy of 5,6,10b-tri­aza­acephenanthrylene

The influence of π–π overlap in centrosymmetric dimers on the aggregation type, single-crystal absorption and fluorescence anisotropy of the new heterocyclic system 5,6,10b-triazaacephenanthrylene is presented.

Populations of the radical transients accompanying bimolecular electron transfer reactions in solutions

Abstract The response functions for the decay of singlet state of the donor (acceptor) in the presence of the acceptor (donor) and the populations of geminal radical pairs (in their singlet and triplet states) formed due to electron transfer in liquid solutions are derived. Computer calculations of the time evolution of the corresponding survival probabilities are presented. Effects related to the spin dynamics of the geminal radical pairs, solvent dielectric friction and diffusion are analyzed for a simple case of the electron shift reaction.