Solvatochromic and rigidochromic fluorescent probes based on D-π-A diaryl ethylene and butadiene derivatives for UV-curing monitoring

This work presents a detailed study of the solvatochromism in both the ground and excited state of different D-pi -A diarylethylene and diarylbutadiene derivatives (4-dimethylamino-4'-nitrostilbene, DMANS; 2-hydroxy-4-diethylamino-4'-nitrostilbene 2OHDEANS, and 4-dimenthylaminophenyl-4'-nitrophenylbutadiene DMANBu). Absorption and emission characteristics indicate the existence of an excited state more relaxed than the Franck-Condon excited state in polar media, due to a twisted intramolecular charge transfer favoured by the co-operative effects of donor and acceptor groups. This feature has been confirmed by the high values of the dipole moments of the excited stares found in this work. A peculiar behaviour has been elucidated fur 2OHDEANS due to the pre-twisted geometry in the ground state shown by this molecule. Moreover, it has been observed that the fluorescence intensity increases as temperature decreases as result of the less effective nonradiative decay process which competes with fluorescence emission. Linear correlations between fluorescence emission area and the reciprocal of the free volume fraction have been obtained during UV-curing of acrylic monomers. Also, fluorescence emission band shifts have been detected during photopolymerisation and the occupied volume by the fluorescent probe appears to be an important parameter that determines the sensitivity of the probe to detect the changes in viscosity and polarity occurring during the W-curing processes. (C) 2001 Elsevier Science Ltd. All rights reserved.