Upon photoexcitation at 345 nm, 3HF exhibits dual emissions with band maxima at around
410 nm and 510 nm (peak positions depend on the solvent), corresponding to the normal and the
photoproduced tautomer respectively (Figure S1 of the supporting information) [3,13-17].
Monitoring at the two respective emission bands, the steady state fluorescence anisotropy values for
both the species of 3HF in different solvents are measured (Table 1). Unprecedented high
fluorescence anisotropies of the normal species (em 410 nm) of the fluorophore have been
recorded in water and other protic solvents; while the tautomer emission (em 520 nm) shows low anisotropy values. Table 1 reflects that with an increase in the protic character of the solvents the
fluorescence anisotropy of the normal form increases, although that of the tautomer species remains
invariant. The low anisotropy values of the tautomer in the studied solvents may be justified by
considering the change in the orientation of the dipole of 3HF on proton transfer as reported by
Bellucci and Coker from their molecular dynamics study [18]. A detailed theoretical treatment is
required to divulge the picture more explicitly. However, for the present purpose we focus on the
normal species only. Nevertheless, this will not lessen the impact of the present work at all since this
is the first systematic report of such high fluorescence anisotropy in conventional protic solvents.
We have extended the study to some hydroxyl derivatives of 3HF, viz., DHF, F and Q (Scheme 1).
The measured anisotropy values monitoring at the normal emission bands of these probes in
different solvents are collected in Table 1. The values are found to go parallel to that of 3HF.