Zbigniew R. Grabowski

Two emitting states of excited p-(9-anthryl)-n,n-dimethylaniline derivatives in polar solvents

Abstract Steady-state and time-resolved fluorescence spectroscopy, as well as the S n ← S 1 absorption spectra of three anthryl derivatives of dimethylaniline, I–III, reveal the existence of two emitting states in all these compounds. The primary locally excited (LE) state of the anthracene moiety relaxes in polar solvents to a highly polar excited state. The relaxation involves most probably an internal rotation between the dialkylaniline donor and the anthracene acceptor structural sub-units, to reach a perpendicular conformation of the twisted internal charge-transfer (TICT) excited state. The intersystem crossing occurs preferentally in the TICT state, being followed by a back electron transfer relaxation to a lower lying LE triplet.

Nitrogen-14 nuclear magnetic resonance of azoles and their benzo-derivatives

Abstract The 14 N NMR spectra of some azoles, diazoles, triazoles, tetrazoles, oxazoles, thiazoles, and some bicyclic benzo-derivatives show a linear relationship between the chemical shifts and the SCF-PPP-MO π-charge densities. The resonances of the N-methyl nitrogen atoms are very characteristic of the type of ring system and may be used for distinguishing between tautomeric forms. The effects of tautomeric equilibria on the appearance of the 14 N spectra are considered. The average excitation energy approximation of the theory of chemical shifts is found to be reliable for the 14 N shifts of pyrrole-type nitrogen atoms in azole ring systems, even if the effect of polarisation in the σ-bonds is ignored.

Theoretical model for the double fluorescence of p-cyano-N,N-dimethylaniline in polar solvents

Abstract The energies of the low lying electronic states of p -cyano-N,N-dimethylaniline (CDMA) in its planar and in 90° twisted conformation have been calculated using the modified INDO method. Solvent effects have been taken into account. The results agree reasonably well with the experimental data and support the hypothesis of the dual fluorescence of CDMA as being related to the intramolecular rotation of the N(CH 3 ) 2 group, with a decisive role of the solvent polarity.

Dual fluorescence of donor-acceptor molecules and the Twisted Intramolecular Charge Transfer (TICT) states

Abstract The hypothesis assigning the dual fluorescence to the formation of a twisted internal charge transfer (TICT) state seems to be proved for p-cyano- and p-(9-anthryl)-dialkylanilines. It appears to be applicable also to numerous other molecules.

Electron transfer in flexible molecules and molecular ions

Electron transfer (ET) occurring in or after electronic excitation in flexibly bonded donor-acceptor (D-A) molecules are accompanied by structural relaxation. In singly bonded D-A molecules often the lltwistedff intramolecular charge transfer (TICT) structures are formed. Evidence for the TICT hypothesis is presented. In some compounds behaviour deviating from the original TICT model is observed; strict applicability of that hypothesis seems to be limited to rather simple D-A molecules. Most deactivation processes involve back ET. Kinetic data on the fluorescence rise and decay seem not always to correspond to the real population kinetics in the case of forbidden transitions in emission; a hot luminescence emitted along the reaction path may distort the curves at short delay times. Contrary to neutral D-A molecules, photophysics of ET in the molecular ions, D-A+, are much less understood: most often, unexpectedly fast radiationless processes (back ET ?) quench the emission. PHOTOINDUCED ELECTRON TRANSFER AND THE STRUCTURAL RELAXATION

Picosecond chemical relaxation of excited p-cyano-n, n-dialkylanilines

Abstract New evidence indicate that the dual fluorescence of p -dimethylamino-benzonitrile (I) originates in a chemical relaxation B * → A * , where B * is a state hidden in the first absorption band A * is a highly polar rotamer with NMe 2 group perpendicular to the ring. New derivatives are synthesised, II–V, including those of enforced coplanar (V) or twisted conformation of NMe 2 group (III, IV). All three transition moments, the main responsible for absorption, b and a , are parallel. The relaxation B * → A * is in CH 3 CN slower than the reorientation of the solvent molecules. A * seems to be related to a “Rydberg” state similar to those of aliphatic amines.

Picosecond isomerisation kinetics of excited p-dimethylaminobenzonitriles studied by oxygen quenching of fluorescence

Abstract Fluorescence quenching with O 2 under high pressure was used for the study of reaction kinetics in the 10 picosecond region. Quenching of the dual fluorescence of 2-methyl-4-dimethylaminobenzonitrile (II) in acetonitrile was analysed. The process B * → A * occurs with a lifetime of 11 ps, while the solvent reorientation correlation time is only 1 ps, which supports the isomerisation mechanism which involves twisting the amino group. The lifetime of A * is 4.5 ns.

Dual and multiple fluorescence mechanisms of p-dimethylaminobenzaldehyde and its trimethylene-bridged double molecule

Abstract p-Dimethylaminobenzaldehyde (DMABA, I) and its N-to-N trimethylene-bridged double molecule (III) exhibit dual or multiple fluorescences. Several mechanisms were proved to be responsible for the long-wave fluorescence bands: intramolecular solvent-assisted relaxation in the excited state (I and III); ground state aggregation at low temperatures (I in nonpolar solvents); excimer formation (III; and I only at high concentrations in some solvents). Intramolecular interaction in the ground state of III prepares an excimer-like structure. The dimer of DMBA aside of its own emission, may relax and emit an excimer-like fluorescence.

Mechanism and kinetics of fluorescence quenching of aromatic hydrocarbons by a stable nitroxyl radical

Abstract The fluorescence quenching of 1,2-benzanthracene and phenanthrene by the nitroxyl radical in ethanol solution at room temperature has been studied by measuring the transient absorption spectra and build-up curves of the triplet-triplet absorption in the presence of the radical. Using a simple kinetic model the rate constant for the catalyzed intersystem crossing reaction has been calculated and compared with the total quenching rate constant obtained from a Stern-Volmer treatment of fluorescence lifetime data. For both hydrocarbons the rate constants for the catalyzed ISC process is found to be lower, above the error limits, than the total quenching rate constant. Since radical hydrocarbon ions are not formed, an energy transfer or a catalyzed internal conversion are the most probable mechanisms for the minor contribution to the quenching kinetics.

Model compounds in study of the photophysical behaviour of carbonyl derivatives of N,N-dimethylaniline

Abstract Carbonyl p-derivatives of N,N-dimethylaniline relax in the excited state to the highly polar (TICT) state with a twist of one of the substituents. Model compounds were synthesized with either the carbonyl or amino group fixed (VIII and IX, resp). It is found that the intramolecular rotation of the amino group is responsible for the TICT state formation. The carbonyl compounds VI-IX show anomalies in their photophysical kinetics (temperature and excitation energy dependencies of the quantum yields).