Excited-State Structure, Vibrations, and Nonradiative Relaxation of Jet-Cooled 5-Fluorocytosine

Lobsiger, Simon; Trachsel, Maria Angela; Den, Sebastian Takuya; Leutwyler, Samuel (2014). Excited-State Structure, Vibrations, and Nonradiative Relaxation of Jet-Cooled 5-Fluorocytosine. Journal of physical chemistry - B, 118(11), pp. 2973-2984. American Chemical Society 10.1021/jp500410s

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The S0 → S1 vibronic spectrum and S1 state nonradiative relaxation of jet-cooled keto-amino 5-fluorocytosine (5FCyt) are investigated by two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm–1 resolution. The 000 rotational band contour is polarized in-plane, implying that the electronic transition is 1ππ*. The electronic transition dipole moment orientation and the changes of rotational constants agree closely with the SCS-CC2 calculated values for the 1ππ* (S1) transition of 5FCyt. The spectral region from 0 to 300 cm–1 is dominated by overtone and combination bands of the out-of-plane ν1′ (boat), ν2′ (butterfly), and ν3′ (HN–C6H twist) vibrations, implying that the pyrimidinone frame is distorted out-of-plane by the 1ππ* excitation, in agreement with SCS-CC2 calculations. The number of vibronic bands rises strongly around +350 cm–1; this is attributed to the 1ππ* state barrier to planarity that corresponds to the central maximum of the double-minimum out-of-plane vibrational potentials along the ν1′, ν2′, and ν3′ coordinates, which gives rise to a high density of vibronic excitations. At +1200 cm–1, rapid nonradiative relaxation (knr ≥ 1012 s–1) sets in, which we interpret as the height of the 1ππ* state barrier in front of the lowest S1/S0 conical intersection. This barrier in 5FCyt is 3 times higher than that in cytosine. The lifetimes of the ν′ = 0, 2ν1′, 2ν2′, 2ν1′ + 2ν2′, 4ν2′, and 2ν1′ + 4ν2′ levels are determined from Lorentzian widths fitted to the rotational band contours and are τ ≥ 75 ps for ν′ = 0, decreasing to τ ≥ 55 ps at the 2ν1′ + 4ν2′ level at +234 cm–1. These gas-phase lifetimes are twice those of S1 state cytosine and 10–100 times those of the other canonical nucleobases in the gas phase. On the other hand, the 5FCyt gas-phase lifetime is close to the 73 ps lifetime in room-temperature solvents. This lack of dependence on temperature and on the surrounding medium implies that the 5FCyt nonradiative relaxation from its S1 (1ππ*) state is essentially controlled by the same ∼1200 cm–1 barrier and conical intersection both in the gas phase and in solution.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Departement of Chemistry and Biochemistry

UniBE Contributor:

Lobsiger, Simon; Trachsel, Maria Angela; Den, Sebastian Takuya and Leutwyler, Samuel

Subjects:

500 Science > 570 Life sciences; biology
500 Science > 540 Chemistry
500 Science

ISSN:

1520-6106

Publisher:

American Chemical Society

Language:

English

Submitter:

Beatrice Niederhauser

Date Deposited:

26 Mar 2015 08:43

Last Modified:

25 Aug 2015 10:27

Publisher DOI:

10.1021/jp500410s

BORIS DOI:

10.7892/boris.65839

URI:

https://boris.unibe.ch/id/eprint/65839

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