Time-Domain Study of the S₃ State of 9-Fluorenone

Abstract

We report a combined gas phase and solution phase study of 9-fluorenone. The structure and dynamics of isolated fluorenone in the S3-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 (1)B2 ← X (1)A1 transition was observed at 32,122 cm(-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 S1/S2-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 S3 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 ππ* character and does not interact with the (3)ππ*.

The solvatochromic response of benzo[a]fluorenone in aprotic solvents compared with benzo[b]fluorenone and 9‐fluorenone

“…As noted above, it has long been suggested that an increased possibility for ISC in nonpolar solvents is the reason for the decreased fluorescence of 9‐FL in nonpolar solvents compared with polar solvents . Recent experiments suggest ISC to a triplet state following red‐edge excitation of 9‐FL in cyclohexane, which also supports the claim of ISC . The similar responses of BFb and 9‐FL in cyclohexane suggest that ISC may also explain the low fluorescence quantum yield and short fluorescence lifetime results for BFb as well.…”

“…In polar solvents, the energy difference between the ground and first excited triplet is similar, while the difference between the ground and second excited triplet increases to approximately 1.1 eV . For several solvents, experimental triplet‐triplet absorption peaks have been observed in the region between 550 and 700 nm (~1.8‐2.3 eV) [2d, 15, 21, 23]. Interestingly, the experimental absorption energies reported in these studies all roughly match the energies differences between the excited n‐π* triplet state and the ground π‐π* triplet state predicted in the present calculations.…”

“…[10][11][12][13][14][15] Recent experiments suggest ISC to a triplet state following rededge excitation of 9-FL in cyclohexane, which also supports the claim of ISC. [21] The similar responses of BFb and 9-FL in cyclohexane suggest that ISC may also explain the low fluorescence quantum yield and short fluorescence lifetime results for BFb as well. The substantially increased emission for 9-FL and BFb in acetonitrile is consistent with the explanation that ISC is forbidden as polarity is increased.…”

“…For both 9-FL and BFb, the character of the first excited singlet is predicted to change from n-π* to π-π* as solvent polarity is increased from the [20] and Gerbich. [21] gas phase. The change for 9-FL happens between cyclohexane and acetonitrile, consistent with the current model for its emission process.…”

An investigation of broadband optical nonlinear absorption and transient nonlinear refraction in a fluorenone-based compound

“…25,26 However, although multiphoton absorption 27,28 in uorenone derivatives has been reported many times, systematic investigations into excited-state associated nonlinear absorption are rare. [29][30][31] Furthermore, most of the research focuses on nonlinear absorption (NLA), little attention has been paid to the ultrafast nonlinear refraction (NLR) in uorenone derivatives.…”