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Excited-State Dynamics of Rhodamine 6G in Aqueous Solution and at the Dodecane/Water Interface
The
excited-state dynamics of rhodamine 6G (R6G) has been investigated
in aqueous solution using ultrafast transient absorption spectroscopy
and at the dodecane/water interface using the femtosecond time-resolved
surface second harmonic generation (SSHG) technique. As the R6G concentration
exceeds ca. 1 mM in bulk water, both R6G monomers and aggregates are
excited to a different extent when using pump pulses at 500 and 530
nm. The excited-state lifetime of the monomers is shortened compared
to dilute solutions because of the occurrence of excitation energy
transfer to the aggregates, which themselves decay nonradiatively
to the ground state with a ca. 70 ps time constant. At the dodecane/water
interface, both monomers and aggregates contribute to the SSHG signal
to an extent that depends on the bulk concentration, the pump and
probe wavelengths, and the polarization of probe and signal beams.
The excited-state lifetime of the monomers at the interface is of
the order of a few picoseconds even at bulk concentrations where it
is as large as several nanoseconds. This is explained by the relatively
high interfacial affinity of R6G that leads to a large interfacial
concentration, favoring aggregation and thus rapid excitation energy
transfer from monomers to aggregates