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Photo-oxa-dibenzocyclooctyne (Photo-ODIBO) undergoes photodecarbonylation under UV excitation to its bright S 2 state, forming a highly reactive cyclooctyne, ODIBO. Following 321 nm excitation with sub-50 fs actinic pulses, the excited state evolution and cyclopropenone bond cleavage with CO release were characterized using femtosecond stimulated Raman spectroscopy and time-dependent density functional theory Raman calculations. Analysis of the photo-ODIBO S 2 CO Raman band revealed multi-exponential intensity, peak splitting and frequency-shift dynamics. This suggests a stepwise cleavage of the two C–C bonds in the cyclopropenone structure that is completed within <300 fs after excitation. Evidence of intramolecular vibrational relaxation on the S 2 state, concurrent with photodecarbonylation, with dynamics matching previous electronic transient absorption spectroscopy, was also observed. This confirms an excited state, as opposed to ground state, photodecarbonylation mechanism resulting in a vibronically excited photoproduct, ODIBO.
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Positive and negative signal and line shape in stimulated Raman spectroscopy: Resonance femtosecond Raman spectra of diphenylbutadiene
Resonance stimulated Raman signal and line shape are evaluated analytically under common electronic/vibrational dephasing and exponential Raman/probe pulse, exp(−|t|/τ). Generally, the signal from a particular state includes contributions from higher and lower electronic states. Thus, with S 0 → S 1 actinic excitation, the Raman signal consists of 15 Feynman diagrams entering with different signs. The negative sign indicates vibrational coherences in S 1 or higher S n , whereas the positive sign reveals coherences in S 0 or S n via S 1 → S n → S m (n < m) coupling. The signal complexity is in contrast to spontaneous Raman with its single diagram only. The results are applied to femtosecond stimulated Raman spectra of trans–trans, cis–trans (ct), and cis–cis (cc) 1,4-diphenyl-1,3-butadiene, the ct and cc being reported for the first time. Upon actinic excitation, the Stokes spectra show negative bands from S 1 or S n . When approaching higher resonances S n → S m , some Raman bands switch their sign from negative to positive, thus, indicating new coherences in S n . The results are discussed, and the measured Raman spectra are compared to the computed quantum-chemical spectra.
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- Award ID(s):
- 1900226
- PAR ID:
- 10345175
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 156
- Issue:
- 8
- ISSN:
- 0021-9606
- Page Range / eLocation ID:
- 084304
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1063/5.0075116
