jp6b11293_si_001.pdf (175.96 kB)
Ultrafast Dynamics of Hydrogen Bond Breaking and Making in the Excited State of Fluoren-9-one: Time-Resolved Visible Pump–IR Probe Spectroscopic Study
journal contribution
posted on 2017-01-12, 00:00 authored by Rajib Ghosh, Aruna K. Mora, Sukhendu Nath, Dipak K. PalitThe fluoren-9-one
(FL) molecule, with a single hydrogen bond-accepting
site (CO group), has been used as a probe for investigation
of the dynamics of a hydrogen bond in its lowest excited singlet (S1) 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
(S0) 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 CO
stretch provides confirmatory evidence of ultrafast photodissociation
of hydrogen bonds in some of the complexes formed in the S0 state. The peak-shift dynamics of the CO stretch bands reveal
two major relaxation pathways, namely, vibrational relaxation in the
S1 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 S1 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.