Femtosecond Intersystem Crossing in the DNA Nucleobase Cytosine Martin Richter,y Philipp Marquetand,z Jes´usGonz´alez-Vazquez,{ Ignacio Sola,{ and Leticia Gonz´alez∗,z yInstitute of Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany zInstitute of Theoretical Chemistry, University of Vienna, W¨ahringerStr. 17, 1090 Vienna, Austria {Departamento de Qu´ımica F´ısica I, Universidad Complutense, 28040 Madrid, Spain E-mail: [email protected] Keywords: DNA photostability, excited Graphical TOC Entry state dynamics, intersystem crossing, spin-orbit coupling, conical intersection Abstract Ab initio molecular dynamics including non- adiabatic and spin-orbit couplings on equal footing is used to unravel the deactivation of cytosine after UV light absorption. Intersys- tem crossing (ISC) is found to compete directly with internal conversion in tens of femtosec- onds, thus making cytosine the organic com- pound with the fastest triplet population cal- culated so far. It is found that close degen- eracy between singlet and triplet states can more than compensate for very small spin-orbit couplings, leading to efficient ISC. The fem- tosecond nature of the intersystem crossing pro- cess highlights its importance in photochem- arXiv:2103.13070v1 [physics.chem-ph] 24 Mar 2021 istry and challenges the conventional view that large singlet-triplet couplings are required for an efficient population flow into triplet states. These findings are important to understand DNA photostability and the photochemistry and dynamics of organic molecules in general. 1 The interaction of DNA and RNA with ra- goes beyond what can be learned from quan- diation, from mobile-phone emissions1 to UV tum chemical calculations alone. wavelengths,2 has enthralled the scientific com- Cytosine presents three tautomers: enol-, munity for years due to its implications in pho- keto- and keto-imine-cytosine. Keto-cytosine todamage.3 Of particular interest is to under- is the biological relevant tautomer found in stand photostability, i.e. the relaxation mech- the DNA's nucleotides linked to the deoxyri- anisms that bring DNA4{6 to the ground state bose sugar moiety and the only one for which before any other photoreaction can occur. This a crystalline structure exists.40 Therefore, here means that, instead of fluorescence or phospho- we focus on keto-cytosine. Several station- rescence, the electronic energy provided upon ary points3,41,42 of keto-cytosine, including two- photoexcitation in DNA is transferred to the 43{48 and three-state49{51 conical intersections nuclear degrees of freedom of the molecular sys- involved in the process of IC have been calcu- tem in different ways. It is precisely the atom- lated with ab initio methods. Time-dependent istic description of these different relaxation calculations have indicated that the dynami- pathways that is still heavily discussed in the cal behavior of cytosine after photoexcitation literature. In the last years it has been clearly is one of the most complicated among nucle- established that excited states of isolated DNA obases, involving delocalization of the excited nucleobases undergo ultrafast internal conver- wave packet and relaxation through multiple sion (IC) allowing for an efficient radiationless competing pathways in the singlet excited state decay towards lower-lying electronic states.7{15 manifold.6,52{54 The possible triplet state for- The role of intersystem crossing (ISC) in the mation via ISC along the internal conversion process of photostability is, however, much less pathway of excited singlet keto-cytosine has discussed,15{19 probably because it is thought been discussed by Merch´anet al.55,56 in the to be a much slower process in comparison to light of quantum chemical calculations. IC20 and also because the quantum yields of Our ab initio molecular dynamic simula- triplet states population in DNA and RNA nu- tions are performed on seven states simultane- cleobases are generally very small and thus dif- ously: four singlets and three triplets. Ener- ficult to access from the experimental point of gies, energy gradients, non-adiabatic and spin- view.10,15,21 We note, however, that ultrafast orbit couplings are computed on-the-fly using time scales for ISC in other organic molecules the state-average Complete Active Space Self- have been experimentally reported or predicted Consistent Field (CASSCF) method.57,58 Fur- before.22{36 ther details are given in the Supporting Infor- In this work we present the first excited state mation (SI). The first singlet excited state, S1, dynamical study of a DNA nucleobase includ- has ππ∗ character at the equilibrium geometry ing singlet and triplet states. Such simulations and it is bright while the states higher in energy, ∗ are done using the newly developed surface- S2 and S3, correspond to dark nπ excitations, hopping method SHARC.37 SHARC stands for i.e. they have negligible oscillator strengths surface hopping including arbitrary couplings. when vertically excited. The order of states at With SHARC one can treat non-adiabatic and equilibrium geometry is not altered when go- spin-orbit couplings (which mediate IC and ing to higher levels of theory that include dy- ISC, respectively) on equal footing. The appli- namical correlation (see Table S2 of SI). How- cability of SHARC to include spin-orbit as well ever, the on-the-fly approach used in this work as dipole couplings is documented in Refs.37{39 prohibits the use of higher level methods such Here, SHARC is employed to investigate the role as CASPT2 and therefore we employ CASSCF. of the triplet states in the deactivation of cy- The Franck-Condon region {from which exci- tosine within the framework of nonadiabatic tations take place{ does not only comprise the molecular dynamics based on ab initio multi- equilibrium geometry but also slightly distorted configurational methods. Such study is nec- geometries. These distortions are due to the essary to provide a mechanistic insight that different vibrations included within the zero- 2 point energy of the system. Because in cyto- that in some cases numbers can be higher than sine rather small deviations of the equilibrium the initial population. Percentages not adding geometry lead to a different ordering of the to 100% are due to minor pathways not indi- state character, the S2 and S3 states can also cated. The decay times are obtained fitting be bright states and contribute to the absorp- the net amount of hops between two particu- tion spectrum (see Refs.6,53 and Fig. S1 of SI). lar states (see Fig. S3 and Table S1 of SI) to an The character of the lowest three excited triplet exponential function. The branching efficien- ∗ states at equilibrium is ππ for T1 and T2, and cies given in % are also graphically indicated by ∗ nπ for T3. A comprehensive report of vertical arrows of different thickness according to their excitation energies at different levels of theory importance. After photo-excitation, which cor- can be found in the SI. responds to time zero in our simulations, the In order to obtain a global picture of the re- population of the ππ∗ is distributed as 13% in laxation mechanisms of keto-cytosine, we have S3, 47% in S2, and 40% in S1, as dictated by the first used initial conditions spanning the whole weight of the oscillator strength of each state. first absorbtion band of the UV spectrum, i.e. Since the character of a state can adiabatically covering excitation energies from ca 4 to 7 eV. change during the simulation, hereafter we shall As explained above, this requires launching tra- refer to the states by its energetic order rather jectories from the first three excited states, S1, than by their character. S2 and S3. Most time-resolved spectroscopic ex- Analyzing the 13% population of S3, 10% re- 9,12,48,59,60 periments in cytosine use a pumping laxes non-adiabatically to the S2 and from there wavelength of 267 nm (4.64 eV), just below the to the S1 within 25 fs. After ca 155 fs, the sys- center of the first absorption band located at tem populates the electronic ground state S0. 260 nm (4.77 eV). In order to narrow the ini- The remaining 3% of the population of S3 deac- tial conditions to the energy range correspond- tivates directly to S1 via a threefold degeneracy 51 ing to the experimental one, we have also ana- S3/S2/S1, as proposed in Ref. This process is lyzed the results (Figure S2 of SI) limited to the calculated to be slower than the previous one, bandwidth 4.75±0.25 eV, just below our theo- with a time constant of 110 fs. Most of the pop- retically predicted first absorption band maxi- ulation in S2 transfers preferably to the lower- mum. Also in this energy range, states S1 to lying electronic states within less than 100 fs. S3 are excited. The results from both energy Also in this case the process of IC is possible via ranges qualitatively agree with each other so a cascade of subsequent S2/S1 and S1/S0 conical that we will limit the discussion to the more intersections, or directly via three-state conical 49,51 general broad range. intersections S2/S1/S0, as proposed in Refs. Figure 1 displays the time evolution of all Both pathways to the ground state are relevant, the state populations and Fig. 2 summarizes in agreement with the time-dependent simula- the most important deactivation paths found in tions of Ref.53 As deduced from the time con- keto-cytosine with SHARC, including decay times stants, the three-state conical intersection path- and branching efficiencies. One should note way is faster (25 fs) than the two-step pathway that because the calculations are done at lev- (25 fs / 155 fs).