Time-resolved evanescent wave induced fluorescence spectroscopy. Part 1.—Deviations in the fluorescence lifetime of tetrasulphonated aluminium phthalocyanine at a fused silica/methanol interface

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Abstract
Evanescent wave induced fluorescence spectroscopy and time-correlated single-photon counting have been combined to provide a method of studying the fluorescence decay profiles of fluorophores at a solid/solution interface. Using these techniques the photophysical properties of tetrasulphonated aluminium phthalocyanine (AlS4Pc) in methanol and water solution have been studied at a fused silica interface. In a poor solvent, such as methanol, AlS4Pc is shown to form an adsorbed monolayer on the fused silica surface and exhibit a fluorescence decay profile that deviates from the simple, first-order, single-exponential kinetics of the bulk solution. By changing the solvent to water, which is a good solvent for AlS4Pc, the surface/bulk differences are not observed. The ability of time-resolved fluorescence spectroscopy to distinguish between concentration and quantum-yield effects on evanescent wave induced fluorescence intensity, is clearly demonstrated.