Time-resolved fluorescence spectroscopy of cationic dyes incorporated in silica matrix by high pressure compaction

The cationic dyes 9-amino acridine (9AA) and auramine were incorporated into silica matrix by sol–gel method, producing transparent monoliths. These fluorochromic materials were converted into powder and compacted at room temperature at 6 GPa to form optical crack-free compact glasses. The dyes incorporated in these three forms (monolith, powder and compact glass) were studied by time-resolved fluorescence spectroscopy in order to obtain information of the excited state dynamics under soft and compressed silica environment. Both dyes have multi-exponential decays in these solid phases. 9AA in monolith and in silica powder shows biexponential decay with components of 8.4–9.0 and 23–24 ns. In compact glass, a fast component of 2.4 ns is detected in addition to the two cited slow components, and it has been ascribed to electronic energy migration among dyes and exciton trapping by dark dimers. For auramine, a dye that fluoresces in rigid matrix by precluding its fast intramolecular twisting and subsequent non-radiative deactivation, the three types of solids enhance the fluorescence signal and give rise to decay processes from picoseconds to nanoseconds in the time scale. In monolith and silica powder, the behavior is practically the same, with three decay components of 0.3, 1.3 and 4.3 ns. However, in compact glass, the two first decay components are shifted to larger values of 0.63 and 2.3 ns, while the third component remains practically constant with a value of 4.1 ns. These results are analyzed with the model of emissive and non-emissive states of auramine under restricted torsional molecular motion.