Excited-state complex formation of perylene and naphthalimides in solutions and films: New materials for molecular thermometers

Synthetic polymers that are designed to change their physical or optical properties in response to environmental stimuli have wide-reaching applications in engineering, biological sensing, and medical fields. The fluorescent polymers can be used in the surface temperature measurement, which has broad applications in aerodynamics and engineering. Dually luminescent systems have become a good alternative over current temperature-sensitive paints since the intensity ratio is measured in these systems. An exciplex is a complex between one compound in the excited-state and the other in the ground-state. Generally, the monomer and the exciplex emission are observed at different wavelengths. The fluorescence intensity ratio of the exciplex to the monomer is temperature and structure dependent. The effect of monomer and polymer structure on the temperature-dependent exciplex formation was the main study focus of this work.Exciplex formation between encapsulated perylene and N-isoproprylacryamide (NIPAM) and N,N-dimethylstyrene (NDS) copolymers was studied in films. Poly(N-isoproprylacryamide) (PNIPAM) is among a class of polymers having a well-characterized lower critical solution temperature (LCST) of 31°C in water. Thus, it is of interest to study temperature-dependent exciplex formation in these polymers. The polymer composition was varied by changing the ratio of NIPAM to NDS. Polymer films containing encapsulated perylene were prepared and studied using steady-state fluorescence spectroscopy. The relative population of exciplex to monomer is temperature dependent. The percent variation of the ratiometric intensity per degree centigrade in poly(N,N-diemthylstyrene) (PNDS) film was found to be about 3%/°C.Perylene derivatives with polymerizable function groups were synthesized for the purpose of covalently attaching perylene into the polymer. The intermolecular exciplexes formed between the perylene derivatives and different anilines were studied by measuring the fluorescence decays at specific temperatures. The decays were fitted to the classic Birk's two-state model. The rate constants for exciplex formation and dissociation and the thermodynamics for exciplex formation were obtained from the fits. The activation barriers for exciplex formation and dissociation for the perylene derivatives with N, N-dimethylaniline were found in the range of 3-40 kJ/mol. The polymers with covalently attached perylene and donors showed a less sensitive temperature response than perylene films. The intramolecular excimer formation in bisnaphthalimide was studied using the similar method at different pHs. The excimers formed in bisnaphthalimide were mainly static excimers at low pHs.