2 research outputs found
Excited-State Structure and Dynamics of KetoāAmino Cytosine: The <sup>1</sup>ĻĻ* State Is Nonplanar and Its Radiationless Decay Is Not Ultrafast
We
have measured the mass- and tautomer-specific S<sub>0</sub> ā
S<sub>1</sub> vibronic spectra and S<sub>1</sub> state lifetimes of
the ketoāamino tautomer of cytosine cooled in supersonic jets,
using two-color resonant two-photon ionization (R2PI) spectroscopy
at 0.05 cm<sup>ā1</sup> resolution. The rotational contours
of the 0<sub>0</sub><sup>0</sup> band
and nine vibronic bands up to +437 cm<sup>ā1</sup> are polarized
in the pyrimidinone plane, proving that the electronic excitation
is to a <sup>1</sup>ĻĻ* state. All vibronic excitations
up to +437 cm<sup>ā1</sup> are overtone and combination bands
of the low-frequency out-of-plane Ī½<sub>1</sub><sup>ā²</sup> (butterfly), Ī½<sub>2</sub><sup>ā²</sup> (boat),
and Ī½<sub>3</sub><sup>ā²</sup> (HāNāC<sup>6</sup>āH twist) vibrations. UV
vibronic spectrum simulations based on approximate second-order coupled-cluster
(CC2) calculations of the ground and <sup>1</sup>ĻĻ* states
are in good agreement with the experimental R2PI spectrum, but only
if the calculated Ī½<sub>1</sub><sup>ā²</sup> and Ī½<sub>2</sub><sup>ā²</sup> frequencies are reduced by a
factor of 4 and anharmonicity is included. Together with the high
intensity of the Ī½<sub>1</sub><sup>ā²</sup> and Ī½<sub>2</sub><sup>ā²</sup> overtone vibronic excitations, this
implies that the <sup>1</sup>ĻĻ* potential energy surface
is much softer and much more anharmonic in the out-of-plane directions
than predicted by the CC2 method. The <sup>1</sup>ĻĻ*
state lifetime is determined from the Lorentzian line broadening necessary
to reproduce the rotational band contours: at the 0<sub>0</sub><sup>0</sup> band it is Ļ = 44 ps, remains
at Ļ = 35ā45 ps up to +205 cm<sup>ā1</sup>, and
decreases to 25ā30 ps up to +437 cm<sup>ā1</sup>. These
lifetimes are 20ā40 times longer than the 0.5ā1.5 ps
lifetimes previously measured with femtosecond pumpāprobe techniques
at higher vibrational energies (1500ā3800 cm<sup>ā1</sup>). Thus, the nonradiative relaxation rate of ketoāamino cytosine
close to the <sup>1</sup>ĻĻ* state minimum is <i>k</i><sub>nr</sub> ā¼ 2.5 Ć 10<sup>10</sup> s<sup>ā1</sup>, much smaller than at higher energies. An additional
nonradiative decay channel opens at +500 cm<sup>ā1</sup> excess
energy. Since high overtone bands of Ī½<sub>1</sub><sup>ā²</sup> and Ī½<sub>2</sub><sup>ā²</sup> are observed in the R2PI
spectrum but only a single weak 2Ī½<sub>3</sub><sup>ā²</sup> band, we propose that Ī½<sub>3</sub><sup>ā²</sup> is a promoting
mode for nonradiative decay, consistent with the observation that
the Ī½<sub>3</sub><sup>ā²</sup> normal-mode eigenvector points toward the āC<sup>6</sup>-puckeredā
conical intersection geometry
Gas-Phase Cytosine and CytosineāN<sub>1</sub>āDerivatives Have 0.1ā1 ns Lifetimes Near the S<sub>1</sub> State Minimum
Ultraviolet
radiative damage to DNA is inefficient because of the
ultrafast S<sub>1</sub> ā S<sub>0</sub> internal conversion
of its nucleobases. Using picosecond pumpāionization delay
measurements, we find that the S<sub>1</sub>(<sup>1</sup><i>ĻĻ</i>*) state vibrationless lifetime of gas-phase keto-amino cytosine
(Cyt) is Ļ = 730 ps or ā¼700 times longer than that measured
by femtosecond pumpāprobe ionization at higher vibrational
excess energy, <i>E</i><sub>exc</sub>. N<sub>1</sub>-Alkylation
increases the S<sub>1</sub> lifetime up to Ļ = 1030 ps for N<sub>1</sub>-ethyl-Cyt but decreases it to 100 ps for N<sub>1</sub>-isopropyl-Cyt.
Increasing the vibrational energy to <i>E</i><sub>exc</sub> = 300ā550 cm<sup>ā1</sup> decreases the lifetimes
to 20ā30 ps. The nonradiative dynamics of S<sub>1</sub> cytosine
is not solely a property of the amino-pyrimidinone chromophore but
is strongly influenced by the N<sub>1</sub>-substituent. Correlated
excited-state calculations predict that the gap between the S<sub>2</sub>(<sup>1</sup><i>n</i><sub>O</sub>Ļ*) and S<sub>1</sub>(<sup>1</sup><i>ĻĻ</i>*) states decreases
along the series of N<sub>1</sub>-derivatives, thereby influencing
the S<sub>1</sub> state lifetime