Jan Najbar

Mechanisms of fluorescence quenching of aromatic molecules by potassium iodide and potassium bromide in methanol–ethanol solutions

Fluorescence quenching of 1-cyanonaphthalene, 2-cyanonaphthalene, 1-methoxynaphthalene, 2-methoxynaphthalene, and fluoranthene by iodide and bromide ions in methanol–ethanol solutions was measured between 293 and 333 K. The dependences of the quenching rate coefficients on the Gibbs free energy (ΔG) of the electron-transfer reaction and temperature are analysed on the basis of: (i) Rips–Jortner and Onuchic theories of electron-transfer reactions in fluid solutions, (ii) numerical evaluation of diffusion limitation of the rate coefficient. The results show clearly that the mechanisms of the fluorescence quenching of aromatic molecules by halide ions are different for negative ΔG(electron-transfer mechanism) and positive ΔG(heavy atom effect). For ΔG < – 0.25 eV, the activation energies of the quenching process show dependences that can be modelled using the Onuchic formula for the electron-transfer rate and the diffusion model for molecular transport.

The influence of iodide ions on the phosphorescence of aromatic hydrocarbons

Abstract The phosphorescence decay functions and spectra of ethanol solutions of six aromatic molecules (AM), naphthalcne- hg , and - d 8 , phenanthrene- h 10 and - d 10 , and chrysene- h 12 and - d 12 were observed in the presence of different KI concentrations at 77 K. Moment analysis of the decay functions shows that the dependence of the AM phosphorescence rate parameter k PT on its distance from a heavy-atom (HA) iodide ion is described by a Dexter-type relation. The HA enhancement of k PT depends on the AM triplet energy E T . The HA-perturbed AM phosphorescence spectrum P (ν) includes not only an intense component, similar to the unperturbed AM spectrum P o (ν), due to the second-order exchange mechanism, but also a structured subspectrum P x (ν) red-shifted by 600–900 cm −1 relative to P o (ν). P x (ν) originates at an asymmetric out-of-plane CH vibrational mode, and it is attributed to a third-order HA perturbation, involving vibronic coupling.

Internal heavy-atom effect on the T1 states of monochloroquinolines and monochloronaphthalenes

Abstract The triplet lifetimes and luminescence quantum yields were measured for seven monochloroquinolines, 2-methyl-4-chloroquinoline, 2-chloro-4-methylquinoline and two monochloronaphthalenes in ethanol at 77 K. The positional dependence of the rate constants for the T 1 → S 0 transitions in the chloroquinolines and chloronaphthalenes were analysed using the Freed—Jortner model of the displaced potential functions for aromatic hydrocarbons. The energy gap dependence of the radiationless transition was found to be very important for the discussion of the internal heavy-atom effect. The relation between theory and experiment is satisfactory when the CH out-of-plane vibrations are chosen as promoting modes for the relaxation of the triplet state for monochloroquinolines.

Fluorescence quenching of derivatives of anthracene by organic electron donors and acceptors in acetonitrile. Electron and proton transfer mechanism

Abstract Fluorescence quenching of anthracene derivatives by organic electron donors (amines) and acceptors was investigated using stationary fluorescence measurements. The dependence of log(kq) on ΔGet shows Rehm-Weller-type behavior. The formation of anion radicals of anthracene, bianthryl, and 9-cyanoanthracene was detected by flash photolysis in systems containing aromatic amines (aniline, 2-bromoaniline, 4-bromoaniline, N,N-dimethylaniline, 4-bromo-N,N-dimethylaniline, N,N-diethylaniline, and 1,4-diazabicyclo[2.2.2]octane). The radical yields decreased and triplet yields increased when bromo derivatives of amines were used as donor quenchers, indicating the heavy-atom effect on spin conversion within radical pairs. The importance of the heavy-atom effect decreased when the energy gap between the charge transfer and molecular triplet states was small. The formation of separated radicals decreased when primary amines were used as quenchers which indicated the existence of an additional path of deactivation of the radical pair. The behavior of amines as quenchers of bianthryl and anthracene is compared with that of inorganic anion quenchers.

LIF excitation spectra of 2,6-dicyanoaniline

Abstract 2,6-Dicyanoaniline (DCA) reveals remarkably different physicochemical and spectral properties than other aniline derivatives. The infrared and Raman spectra for DCA in solid state and laser induced fluorescence (LIF) excitation spectra of vacuum jet cooled molecules have been investigated. The ground S 0 and the S 1 (B 2 ) excited state geometries, charge distributions and vibrational frequencies of DCA have been evaluated by ab initio calculations. The Dushinsky matrices have been applied for the mode correlation and assignments of the transition frequencies in LIF excitation spectra of DCA(NH 2 ) and DCA(ND 2 ). The intensity distributions in LIF excitation spectra were modeled. The dominant vibronic bands in the excitation spectra are connected with motions of the adjacent cyano- and amino groups. The transitions involving inversion motion of the amino group have low intensities.

LIF excitation spectra of jet-cooled 3,5-dicyanoaniline

The infrared and Raman spectra for crystalline 3,5-dicyanoaniline (3,5-DCA) and the laser induced fluorescence (LIF) excitation spectra of vacuum jet cooled molecules have been investigated. The ground S0(A1) and excited S1(B2) state geometries, charge distributions and vibrational frequencies of 3,5-DCA have been evaluated by ab initio calculations. The Dushinsky matrices have been used for the mode correlation and assignments of the transition frequencies in LIF excitation spectra of 3,5-DCA (−NH2) and 3,5-DCA(−ND2) as well as the IR and Raman spectra for deuterated species. It has been found that the transitions I20 involving inversion motion of the amino group in 3,5-DCA show remarkable intensities suggesting substantial change in the potential energy function upon electronic excitation for the out-of-plane motion of the hydrogen atoms in the amino group. The barrier height for the inversion in the ground electronic state of 3,5-DCA seems to be the same order as the zero point energy. For the lowest excited electronic state the potential function for the inversion has single minimum but shows substantial anharmonicity. The intensity distribution in the LIF excitation spectrum has been modeled using multidimensional Franck–Condon factors. Geometry changes for S0 → S1 excitation of 3,5-DCA are discussed.

LIF excitation spectra of o- and m-cyanoanilines

LIF excitation spectra of 2-cyanoaniline (2-CA) and 3-cyanoaniline (3-CA) in their hydrogen (–NH2) and deuterium (–ND2) forms are presented and discussed. Analyses of harmonic motions for the three mono cyanoanilines are presented, using the results of ab initio (HF/6-31++G** and CIS/6-31++G**) quantum mechanical calculations for ground and excited electronic states, as well as IR and Raman frequencies. Previous assignments of IR and Raman spectra of the cyanoanilines are compared with the ab initio calculations. Potential energy changes upon electronic excitation, to the lowest singlet state, for the cyanoanilines are interpreted in terms of harmonic frequencies, displacements of normal modes and Dushinsky rotation matrices.

External heavy atom effect on decay of the triplet state of aromatic hydrocarbons. II. The decay functions of phosphorescence and of ESR signals of triphenylene in the presence of iodide ions

The decay functions of phosphorescence and ESR signals of triphenylene in the presence of KI in ethanol at 77 K are discussed. It was found that iodide ions are more effective in inducing radiative transition T1 → S0 than radiationless transition T1 ⇝ S0. An active sphere model gives about 9 A for the radius of the sphere of interaction between the aromatic molecule and iodide ion. Using the Dexter formula for the distance dependence of the radiative rate constant T1 → S0 we obtain R0 = 10 A and γ = 9.5. The external heavy atom effect for the triphenylene-I- system may be explained in terms of the intensity borrowing from the perturber due to exchange and spin-orbit interactions.

External heavy atom effect on decay of the triplet state of aromatic hydrocarbons. I. The decay functions of phosphorescence and of the population of the triplet state

Abstract The derivation of the decay functions of the phosphorescence and population of the triplet states of aromatic molecules in the presence of perturbers, under steady-state and flash excitation, taking into account the influence of perturbers on three rate constants: S1 ⇝ Ti, kTM; T1 → S0, kPT; ⇝ S0, kGT, valid in rigid media has been presented.

Solvent dielectric relaxation properties and the external heavy atom effect in the time-resolved fluorescence quenching of anthracene by potassium lodide and potassium thiocyanate in methanol and ethanol

Steady-state fluorescence quenching and the time-resolved fluorescence decay profiles of anthracene in the presence of iodide and thiocyanate ions in methanol and ethanol have been measured in the temperature range between 253 and 333 K. The Rips–Jortner and Onuchic theories of electron transfer are applied for data analysis. We are able to fit our experimental data with reasonable values of parameters characterizing both chemical reaction and molecular transport. Analysis of the experimental data indicates the importance of solvent dielectric relaxation properties for the fluorescence quenching. In the case of iodide ions as quencher the external heavy atom effect seems also to play an important role in the quenching process.