Intersystem Crossing and Characterization of Dark States in the Pyrimidine Nucleobases Uracil, Thymine, and 1-Methylthymine

Abstract

The ground and low-lying excited states of the pyrimidine nucleo bases uracil, thymine, and 1-methylthymine have been characterized using ab initio coupled-cluster with approximate doubles (CC2) and a combination of density functional theory (DFT) and semiempirical multireference configuration interaction (MRCI) methods. Intersystem crossing rate constants have been determined perturbationally by employing a nonempirical one-center mean-field approximation to the Breit-Pauli spin-orbit operator for the computation of electronic coupling matrix elements. Our results clearly indicate that the S(2)((1)pi-->pi*)-->T(2)((3)n-->pi*) process cannot compete with the subpicosecond decay of the S(2) population due to spin-allowed nonradiative transitions, whereas the T(1)((3)pi-->pi*) state is populated from the intermediate S(1)((1)n-->pi*) state on a subnanosecond time scale. Hence, it is very unlikely that the S(1)((1)n-->pi*) state corresponds to the long-lived dark state observed in the gas phase.

Intersystem crossing rates of S1 state keto-amino cytosine at low excess energy

“…In previous work on thymine and uracil, 34 we had combined potential energy surfaces from CC2 calculations and spin-orbit matrix elements (SOMEs) obtained from DFT/MRCI wave functions to determine ISC rate constants. This procedure seemed inappropriate here because the SCS-CC2 and the DFT/MRCI calculations gave substantially different electronic structures of the excited states.…”

“…32,33 At least for vibrationally cold S 1 cytosine, this result contradicts the recent experimental findings by Lobsiger et al 15,16 For the pyrimidine bases uracil and thymine, Etinski et al carried out combined coupled-cluster and DFT/MRCI calculations to determine ISC rate constants. 34 They predicted ISC to play an important role in the electronic relaxation from the initially excited 1 ππ * state in the gas phase. This was later confirmed experimentally.…”

Multi-photon ionization and fragmentation of uracil: Neutral excited-state ring opening and hydration effects

“…They were not able to measure uracil and attributed this to the limitations of their pulsed valve as opposed to the molecule's photo-physical properties. Etinski et al 19 theoretically characterized the lowest lying electronic states of uracil and performed calculations indicating that the T 1 (ππ*) state is populated from the S 1 (nπ *) state on a sub-nanosecond time scale. However, single photon ionization from this triplet state is unlikely in the present energy range (5.64-4.60 eV) in view of its low vertical excitation energy (3.65 eV from Abouaf et al's EELS measurements cited as a private communication by Nguyen et al 20 ) compared with the ionization energy of uracil (9.15 ± 0.03 eV 21 ).…”

Characterizing the dark state in thymine and uracil by double resonant spectroscopy and quantum computation

“…This state was also observed for methylated derivatives of uracil and thymine 29,30 . Several candidates for the dark state have been suggested, including the vibrationally "hot" ground state, the S 1 state of 1 (nπ*) character, and the T 1 state of 3 (ππ*) or 3 (nπ*) character [27][28][29][30][31][32][33] . The involvement of different tautomeric forms of thymine was ruled out by Schultz and co-workers on the basis of theoretical calculations 28 .…”

“…The involvement of different tautomeric forms of thymine was ruled out by Schultz and co-workers on the basis of theoretical calculations 28 . According to the current understanding of the photophysics of thymine, the 3 [30][31][32] . Etinski et al 32 provided support of the triplet state based on the interpretation of time resolved infrared spectra in acetonitrile solution 33 , in particular the peak at 1500 cm -1 , resulting from a combination of the C4-C5 stretching and the N1H wagging motions, and the peak near 1350 cm -1 due to the ring deformation vibration.…”