Karl-Heinz Grellmann

Reinterpretation of the anomalous fluorescense of p-n,n-dimethylamino-benzonitrile

Abstract Two fluorescense bands of p -N,N-dimethylamino-benzonitrile (DAB, I) have previously been ascribed to two emitting states, the strongly polar 1 L b and the less polar 1 L b , or to the presence of an excimer. However, the parallel polarization of both emission bands in derivatives II and III, the effect of substituents, especially the steric effect on the dimethylamino group, and the lack of concentration effects in a broad concentration range, invalidate these hypotheses. We tentatively assign the emissions to the existence of two excited species differing in polarity and in the orientation of the —N(CH 3 ) 2 group: a lone pair perpendicular or parallel to the plane of benzene ring; in the latter case π * y orbitals of the —CN group seem to be involved in the excitation.

Mechanism of the photochemical cis⇌trans isomerization of thioindigo and 6,6′-diethoxy-thioindigo in solution

Abstract The light induced isomerization of thioindigo (I) and 6,6′-diethoxy-thioindigo(II) has been investigated by means of nanosecond flash photolysis and steady state methods. A mechanism is proposed in which a common triplet state generated from both the trans and cis isomers is responsible for the isomerization. Quantum yields at different oxygen concentrations

Determination of decay constants with a sampling flash apparatus. The triplet state lifetimes of anthracene and pyrene in fluid solutions

Abstract A sampling flash apparatus for recording weak transient absorptions in the microsecond range is described. Two methods for the evaluation of first- and second-order rate constants from mixed-order plots are briefly discussed. The concentration dependence of the lifetime of anthracene in its excited triplet state has been investigated in cyclohexane at room temperature. It is assumed that the observed decrease in triplet lifetime with increasing anthracene concentration is due to impurity quenching and not due to triplet excimer formation.

The triplet state of anthracene photodimers and the wavelength dependence of the photodissociation process

Abstract Photodimers of anthracene and 9-methylanthracene photodissociate around 300 K mainly via the excited singlet state, around 77 K exlusively via the triplet state. As an intermediate an electronically excited “tight complex” (X s ) is proposed, of which only a small fraction rearranges to an excimer. The quantum yield of excimer and triplet formation is wavelength dependent.

Triple complex formation and intersystem crossing in the excited state

Abstract The fluorescence properties and the intersystem crossing yield of the system anthracene-diethylaniline in nonpolar solvents have been investigated at different temperatures. Fluorescence transformation to the hetero-excimer 1 (A − D + ) and subsequent formation of an excited triple complex 1 (A − D + D) accounts for the observed temperature dependence of intersystem crossing and fluorescence yields as well as the lifetime of the hetero-excimer. Fast intersystem crossing from a vibrationally non-relaxed hetero-excimer can be ruled out.

Mechanism of the photodissociation of the 9-methylanthracene photodimer

Abstract The photodissociation of the 9-methylanthracene photodimer (A 2 ) has been investigated in a non-polar solvent between 300 K and 77 K by means of steady state and flash experiments. A reaction scheme is proposed according to which at temperatures above 220 K photodissociation originates in a non-relaxed excited singlet state of the dimer whereas below 200 K it occurs via intermediate excimer formation. Around 77 K the excited triplet state, presumably, is the main source of dissociation.

Tunnel effect on the kinetics of a sigmatropic shift

Abstract Non-linear Arrhenius plots and large kinetic isotope effects observed in sigmatropic hydrogen shifts of photochemically generated transient di- and hexa-hydrocarbazoles are interpreted as tunnel contributions. An Eckart barrier was used to calculate the quantum-mechanical transmission probabilities of the tunneling protons.

The photoionization of perylene in acetonitrile solutions

Abstract Two mechanisms are responsible for the flash induced ionization of perylene in acetonitrile: a bimolecular reaction between an excited and an unexcited perylene molecule, and, at shorter exciting wavelengths, direct dissociation of some higher excited singlet state.

Tunnel effect on the kinetics of hydrogen shifts. The enol-ketone transformation of 2′-methylacetophenone

Abstract A large isotope effect on the enol—keto transition of 2′-methylacetophenone and a temperature-dependent activation energy for this process have been observed in flash experiments. A tunnel process and the existence of two metastable rotamers of the enol are proposed to describe the kinetcis of the reketonization process.

The photophysics and photochemistry of pentahelicene

Pentahelicene(II) undergoes photocyclodehydrogenation (PCDH) to benzoperylene(IV) when irradiated with UV in the presence of iodine at temperatures as low as −100°C. The quantum yield varies with the concentration of iodine, reaching a maximum of about 0.15 at [I2] = (2 – 3) × 10−4 M. Flash studies in the microsecond and nanosecond ranges failed to provide evidence for the existence of the postulated primary photocyclization product III in the ground state. Two transients were observed: the first was identified as triplet pentahelicene (3II*), while the second, which was formed from the first, was tentatively assigned to the triplet state of dihydrobenzoperylene(III), i.e. the cyclization is postulated to take place at the triplet level. The sequence 1II → 1II* → 3II* → 3III* → “products” was investigated in the temperature range 70 to −180 °C. In the presence of iodine 3II* is partially quenched to 1II, while 3III* is dehydrogenated. The competition between these two opposing effects of iodine is responsible for the observed variation of the quantum yield with [I2]. With benzopentahelicene(V) which does not undergo PCDH, only one transient, triplet V, was observed.