Ultrafast Dynamics of Hydrogen Bond Breaking and Making in the Excited State of Fluoren-9-one: Time-Resolved Visible Pump-IR Probe Spectroscopic Study

The fluoren-9-one (FL) molecule, with a single hydrogen bond-accepting site (C=0 group), has been used as a probe for investigation of the dynamics of a hydrogen bond in its lowest excited singlet (S-1) state using the subpicosecond time resolved visible pump-IR probe spectroscopic technique. In 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), a strong hydrogen bond-donating solvent, the formation of an FL alcohol hydrogen-bonded complex in the ground electronic (S-0) state is nearly complete, with a negligible concentration of the FL molecule remaining free in solution. In addition to the presence of a band due to the hydrogen-bonded complex in the transient IR spectrum recorded immediately after photoexcitation of FL in HFIP solution, appearance of the absorption band due to a free C=0 stretch provides confirmatory evidence of ultrafast photodissociation of hydrogen bonds in some of the complexes formed in the So state. The peak-shift dynamics of the C=0 stretch bands reveal two major relaxation pathways, namely, vibrational relaxation in the S-1 state of the free FL molecules and the solvent reorganization process in the hydrogen-bonded complex. The latter process follows bimodal exponential dynamics involving hydrogen bond-making and hydrogen bond-reorganization processes. The similar lifetimes of the S, states of the FL molecules, both free and hydrogen-bonded, suggest establishment of a dynamic equilibrium between these two species in the excited state. However, investigations in two other weaker hydrogen bond donating solvents, namely, trifluoroethanol (TFE) and perdeuterated methanol (CD3OD), reveal different features of peak-shift dynamics-because of the prominence of the vibrational relaxation process over the hydrogen bond-reorganization process during the early time.