Jerzy Herbich

Radiative and radiationless depopulation of the excited intramolecular charge transfer states: Aryl derivatives of aromatic amines

Abstract Photoinduced electron transfer in a series of aryl derivatives of aromatic amines has been studied in solutions. The intramolecular charge transfer (CT) leads to a large Stokes shift and large dipole moment in the fluorescent state. The charge recombination (CR) processes are observed by means of the luminescence quantum yields and kinetic measurements. The transition moment of the radiative depopulation of the CT state decreases with the increase of the steric hindrance to coplanarity between donor and acceptor entities and with increasing solvent polarity. The radiationless CR is controlled by two competitive mechanisms: internal conversion to the Franck-Condon ground state and intersystem crossing to the triplet manifold.

Excited charge transfer states in 4-aminopyrimidines, 4-(dimethylanilino)pyrimidine and 4-(dimethylamino)pyridne

Abstract This paper presents a comparative study of the photoinduced electron transfer in a series of donor-acceptor compounds and their hydrogen-bonded complexes in solution. 4-(Dimethylamino) pyridine III, similarly to 4-(dimethylamino) pyrimidine I and 4-(dimethylamino)-5-methylpyrimidine II and contrary to 4-(N,N-dimethylanilino)pyrimidine IV, shows dual luminescence in a sufficiently polar and mobile environment. The results of steady-state and kinetic investigations as well as quantum chemical calculations of I–III fit well into the twisted intramolecular charge transfer (TICT) state model. On the other hand, the results suggest an enhanced planarity of the ICT fluorescent state of IV. INDO/S calculations confirm the large probability of the allowed radiative transitions in the latter compound. Photophysics of all the compounds under study is modified by hydrogen bonding: fluorescence quantum yields are strongly reduced in the presence of alcohols.

Radiative electron transfer in aryl derivatives of dimethylanilines

Abstract Radiative electron transfer (RET) has been studied in six members of the series of aryl derivatives of aromatic amines by means of the solvent and temperature effects on the position of CT fluorescence maxima, the half-widths, quantum yields (Φ f ) and excited-state depopulation kinetics (τ f ). The radiative rate constant ( k f =Φ f /τ f ) for all compounds decreases considerably with increasing solvent polarity and, contrary to our expectations, the fluorescence is not thermally activated. Moreover, k f values decrease markedly with the increase of the energy of the Franck-Condon (FC) ground state reached upon CT emission. The reduction of the half-width of the CT fluorescence spectra with lowering of temperature, as well as the corresponding solvatochromic effects, allow us to determine the outer (λ o ) and inner (λ i ) reorganization energies in terms of the Marcus theory in the inverted region. The results strongly suggest the nonorthogonal conformation of the CT fluorescent states of the compounds under study. The lowest excited state potential energy minimum of these compounds corresponds likely to the more similar values of the twist angle, Θ F ≈55°–70°, between donor and acceptor moieties than those in the ground state. The transition dipole moment values of the radiative depopulation of the CT state are unexpectedly high as compared with those of the corresponding arenes. The mechanism of the radiative electron transfer (RET) is briefly discussed.

Photoinduced intramolecular electron transfer in 4-dimethylaminopyridinesDedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays.

Contrary to 4-(dimethylamino)pyridine (DMAP), which revealed solvent-dependent dual fluorescence, its sterically hindered ortho-methylated derivatives: 3-methyl-4-(dimethylamino)pyridine and 3,5-dimethyl-4-(dimethylamino)pyridine show single charge transfer fluorescence band. The electronic structures of the lowest excited states and molecular geometries were investigated by stationary and kinetic absorption and fluorescence spectroscopy as well as time-dependent density functional response theory (TD-DFT) and semiempirical (INDO/S) calculations. The results can be interpreted in terms of the TICT state model.

Phosphorescent intramolecular charge transfer triplet states

Abstract A comparative study of the electronic structure of the lowest excited triplet state T 1 is presented for a series of N-bonded donor-acceptor derivatives of 3,6-di-tert-butylcarbazole containing benzonitrile, nicotinonitrile or various dicyanobenzenes as electron acceptor. Solvent, temperature and concentration effects on phosphorescence, measurements of luminescence anisotropy and lifetimes, and ESR investigations of selected compounds show the dependence of the electronic structure of their T 1 states on the electron affinity of the acceptor moiety and point to the 3 CT character of the emitting triplet states in 3,6-di-tert-butylcarbazol-9-yl dicyanobenzenes.

Quenching of fluorescence of 2-(2′-pyridyl) indoles upon complexation with alcohols

Abstract Formation of a cyclic, doubly hydrogen-bonded 1:1 complex of alcohols with 2,2′-pyridylindole and its 3,3′-methylene-bridged derivatives leads to an efficient radiationless depopulation of the lowest excited singlet state. Cyclic structure is crucial for this process, since no quenching is observed in the acyclic alcohol complexes of 2,2′-pyridylindole.

A simple route to alloyed quaternary nanocrystals Ag-In-Zn-S with shape and size control.

A convenient method of the preparation of alloyed quaternary Ag-In-Zn-S nanocrystals is elaborated, in which a multicomponent mixture of simple and commercially available precursors, namely, silver nitrate, indium(III) chloride, zinc stearate, 1-dodecanethiol, and sulfur, is used with 1-octadecene as a solvent. The formation of quaternary nanocrystals necessitates the use of an auxiliary sulfur precursor, namely, elemental sulfur dissolved in oleylamine, in addition to 1-dodecanethiol. Without this additional precursor binary ZnS nanocrystals are formed. The optimum reaction temperature of 180 °C was also established. In these conditions shape, size, and composition of the resulting nanocrystals can be adjusted in a controlled manner by changing the molar ratio of the precursors in the reaction mixture. For low zinc stearate contents anisotropic rodlike (ca.3 nm x 10 nm) and In-rich nanocrystals are obtained. This is caused by a significantly higher reactivity of the indium precursor as compared to the zinc one. With increasing zinc precursor content the reactivities of both precursors become more balanced, and the resulting nanocrystals are smaller (1.5-4.0 nm) and become Zn-rich as evidenced by transmission electron microscopy, X-ray diffraction, and energy-dispersive spectrometry investigations. Simultaneous increases in the zinc and sulfur precursor content result in an enlargement of nanocrystals (2.5 to 5.0 nm) and further increase in the molar ZnS content (up to 0.76). The prepared nanoparticles show stable photoluminescence with the quantum yield up to 37% for In and Zn-rich nanocrystals. Their hydrodynamic diameter in toluene dispersion, determined by dynamic light scattering, is roughly twice larger than the diameter of their inorganic core.

Radiative and nonradiative electron transfer in donor–acceptor phenoxazine and phenothiazine derivatives

Abstract Radiative and nonradiative electron transfer (exemplified by the CT fluorescence and nonradiative charge recombination process in the singlet manifold, respectively) in a series of donor–acceptor phenoxazine and phenothiazine derivatives is reported. An analysis of the CT fluorescence leads to the quantities relevant for the radiative electron transfer in the Marcus inverted region. Using a nonadiabatic theory of electron transfer and the latter parameters, the rate constants for nonradiative electron transfer can be predicted. Electronic coupling elements V 0 between the 1 CT state and the ground state obtained from the radiative rates are in agreement with those calculated from the simple LCAO MO model which assumes that V 0 are mainly determined by the interactions between the atoms forming the A–D bond.

Electronic and molecular structure of charge transfer singlet states: 4-(9-anthryl)julolidine and 4-(9-acridyl)julolidine

Abstract A quantitative analysis of the charge transfer (CT) absorption and emission spectra of 4-(9-anthryl)julolidine and 4-(9-acridyl)julolidine and the determination of the corresponding electronic transition dipole moments have made it possible to estimate the electronic coupling elements V 0 and V 1 between the 1 CT state and the ground state S 0 , or the 1 LE states excited locally within the acceptor subunit, respectively. To describe the excited 1 CT state properties, the significant contributions of both of the above interactions together with the solvent induced changes in V 0 and V 1 have to be taken into account. In low polarity solvents the conformation of these compounds in the fluorescent 1 CT state is more planar than that in the ground state (and in the unrelaxed Franck-Condon CT excited state), whereas in a highly polar environment the compounds probably do not undergo any significant conformational changes accompanying the excited state charge separation.

Radiative electron transfer in planar donor–acceptor quinoxaline derivatives

Abstract Synthetic and spectral studies have been performed for a family of electron donor–acceptor (D-A) quinoxaline derivatives possessing an aromatic amine as an electron donor. A photophysical behaviour of the compounds with an internal degree of freedom for internal D-A rotation and their rigid analogues with a fixed planar conformation appears to be very similar. Electronic transition dipole moments related to the charge-transfer (CT) absorption and fluorescence are determined by both the direct interactions between the 1 CT and ground states and by the contributions from the locally excited configurations. The radiative properties of the D-A systems under study can be explained in terms of the simple model which assumes that the electronic coupling elements are mainly determined by the interactions between the atoms forming the A-D bond.