Damped Oscillations in UV−Vis Spectra of Pentazadiene Monomers and Polymers after Laser Photolysis

Abstract

The photochemical decomposition of substituted 1,5-diaryl-3-alkyl-pentazadiene monomers and polymers induced by UV irradiation with a pulsed excimer laser was monitored by UV-vis spectroscopy. The net photochemical reaction proceeds in an overall one-step pathway AfB. Quantum yields for the laser decomposition are determined to be e10%. An oscillating behavior of the absorption was found during the dark period following the irradiation. The temperature dependence of this dark reaction has been studied. The oscillatory behavior is discussed in the light of current theories concerning instabilities in photochemical systems.

Convection-Induced Absorption Oscillations in a Cuvette after Irradiation of a Pentazadiene Solution by Laser Pulses

“…On this background, observations of unusual absorbance oscillations occurring after irradiation of a pentazadiene solution by laser pulses triggered our interest and our attempts to interpret them on a photochemical basis. 2 However, as we will show in this paper, the observed damped oscillations were caused by convection and not by photochemical reactions. We would like to add that convection, especially when combined with polymerization reactions, interfacial tension, and diffusion, can create a large variety of interesting phenomena that are investigated by different groups.…”

“…Due to biological implications, the interest for new and different chemical oscillators is still very high. On this background, observations of unusual absorbance oscillations occurring after irradiation of a pentazadiene solution by laser pulses triggered our interest and our attempts to interpret them on a photochemical basis . However, as we will show in this paper, the observed damped oscillations were caused by convection and not by photochemical reactions.…”

“…The absorption band of the pentazadienes is in the range of common excimer laser wavelengths, i.e., XeCl (308 nm) and XeF (351 nm). Various pentazadiene model compounds and polymers have been investigated to elucidate the UV-induced decomposition pathway, the thermolysis mechanism, and the quantum yield of decomposition …”

Multi-photon UV photolysis of gaseous polycyclic aromatic hydrocarbons: Extinction spectra and dynamics

“…Despite similarities concerning the necessity for convection currents between the work in Ref. 89 and the dynamic behavior described here, the types of oscillations in this work are more diverse, with often multiple components oscillating, allowing only a qualitative description of the phenomenon. The chamber walls in the present case act as inhibitor for oscillations and the efficiency of damping cannot easily be defined, whereas in the cuvette, the damping of oscillations was due to diffusive mixing.…”

“…Damped oscillations, with periods of 30-60 s, of the absorption of 1,5-diaryl-3-alkyl-pentazadiene monomers and polymers, in a temperature controlled cuvette, induced by pulsed UV irradiation (at 308 nm) has been reported. 89 Absorption inhomogeneities produced by the laser irradiation combined with a convection current induced by the temperature stabilizing system resulted in the oscillations. 90 The period of oscillations could be determined through the stabilization temperature and the boundary conditions for the liquid phase study were generally better defined.…”