Time-Domain Study of the S sub(3) State of 9-Fluorenone

We report a combined gas phase and solution phase study of 9-fluorenone. The structure and dynamics of isolated fluorenone in the S sub(3)-state were studied by resonant enhanced multiphoton ionization with picosecond pulses in a free jet of molecules excited between 285 and 312 nm. Ionization was performed with a second ps-pulse at 351 nm. The electronic spectrum is structured, and the origin of the C super(1)B sub(2) arrow left X super(1)A sub(1) transition was observed at 32 122 cm super(-1). Several vibrational fundamentals appear in the spectrum. In the gas phase we observe a biexponential decay, which suggests an internal conversion to the coupled S sub(1)/S sub(2)-state within 10-40 ps. A further decay that is assigned to intersystem crossing was found to be longer than 500 ps. In addition to the gas phase measurements, we studied the photophysics of 9-fluorenone in cyclohexane by femtosecond-time-resolved transient absorption spectroscopy and observed very similar dynamics upon excitation to the S sub(3) state: It deactivates within 8-11 ps by internal conversion, followed by intersystem crossing within 120-150 ps, forming a long-lived triplet state. Experiments in acetonitrile, however, showed marked differences. Intersystem crossing is ineffective in polar solvents because the lowest excited singlest state is of pi pi * character and does not interact with the super(3) pi pi *.