jp8b04417_si_001.pdf (338.63 kB)
Excited-State Proton Transfer from the Photoacid 2‑Naphthol-8-sulfonate to Acetonitrile/Water Mixtures
journal contribution
posted on 2018-07-06, 00:00 authored by Oren Gajst, Luís Pinto da Silva, Joaquim C. G. Esteves da Silva, Dan HuppertSteady-state
and time-resolved fluorescence techniques were used
to study excited-state proton transfer (ESPT) to water of the reversible
photoacid 2-naphthol-8-sulfonate (2N8S) in acetonitrile/water mixtures.
In acetonitrile-rich mixtures, up to χwater ≤
0.12, we found a slow ESPT process on the order of nanoseconds. At
χwater ≈ 0.15, the RO– fluorescence
band intensity is at the minimum, whereas at χwater ≈ 0.030, it is at the maximum. The steady-state fluorescence
spectra of these mixtures show that the intensity of the RO– fluorescence band at χwater ≈ 0.030 is about
0.24 of that of the ROH band. We explain this unusual phenomenon by
the presence of water clusters that exist in the acetonitrile-rich
CH3CN/H2O mixtures. We propose that a water
bridge forms between the 2-OH and 8-sulfonate by preferential solvation
of 2N8S, and this enables the ESPT process between the two sites of
the molecular structure of 2N8S. In mixtures of χwater ≥ 0.25, the ESPT process takes place to water clusters in
the bulk mixture. The higher the χwater in the mixture,
the greater the ESPT rate constant. In neat water, the rate constant
is rather small, 4.5 × 109 s–1.
TD-DFT calculations show that a single water molecule can bridge between
2-OH and 8-sulfonate in the excited state. The activation energy for
the ESPT reaction is about 9 kcal/mol, and the RO–(S1) species is energetically above the ROH(S1) species by about 1.6 kcal/mol.