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Abstract:
The excited state dynamics of the dye ATTO 465, a well-known fluorescence marker for biological
applications, have been characterized in various solvents including THF, ethanol, methanol, water and
the highly polar protic ionic liquid 2-hydroxyethylammonium formate (2-OH-EAF) by combining results
from time-correlated single-photon counting (TCSPC) and ultrafast pump–supercontinuum probe (PSCP)
spectroscopy as well as steady-state absorption and fluorescence. In water, 2-OH-EAF and two
fluorinated alcohols, there is a pronounced blue-shift and broadening of the S0 - S1 absorption band
and also a larger Stokes shift than in the other solvents, indicating a particular influence of hydrogenbonding
interactions. S1 lifetimes from TCSPC at 25 1C range from 3.3 ns to 5.6 ns. An unusual increase
in the S1 lifetime with temperature is observed for ethanol and methanol, however water behaves in
the opposite way. The behavior can be tentatively explained by a solvent- and temperature-dependent
‘‘proximity effect’’, where coupling of the close-lying S1 and S2 states influences the intramolecular
relaxation rate of the dye. In addition, temperature-dependent complex equilibria of ATTO 465 with
solvent molecules may influence the measured lifetimes. Several excited-state absorption (ESA)
transitions are identified in the PSCP spectra, which are in good agreement with the position of the UV
bands in the steady-state absorption spectra. Small shifts of the stimulated emission and ESA bands are
consistent with solvation dynamics in the excited electronic state. An additional B16 ps component in
water, visible over the entire spectral range, is tentatively ascribed to a fast IC channel which is accessed
by a fraction of ATTO 465 molecules.
