Photodynamics Simulations of Thymine: Relaxation into the First Excited Singlet State

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Chemistry Publications Chemistry Photodynamics Simulations of Thymine: Relaxation into the First Excited Singlet State Jaroslaw J. Szymczak University of Vienna Mario Barbatti University of Vienna
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Chemistry Publications Chemistry Photodynamics Simulations of Thymine: Relaxation into the First Excited Singlet State Jaroslaw J. Szymczak University of Vienna Mario Barbatti University of Vienna Jason T. Soo Hoo Siena College Jaclyn A. Adkins Northwest Missouri State University Theresa Lynn Windus Iowa State University, See next page for additional authors Follow this and additional works at: Part of the Chemistry Commons The complete bibliographic information for this item can be found at chem_pubs/918. For information on how to cite this item, please visit howtocite.html. This Article is brought to you for free and open access by the Chemistry at Iowa State University Digital Repository. It has been accepted for inclusion in Chemistry Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact Photodynamics Simulations of Thymine: Relaxation into the First Excited Singlet State Abstract Ab initio nonadiabatic dynamics simulations are reported for thymine with focus on the S2 S1deactivation using the state-averaged CASSCF method. Supporting calculations have been performed on vertical excitations, S1 and S2 minima, and minima on the crossing seam using the MS-CASPT2, RI-CC2, MR-CIS, and MR-CISD methods. The photodynamical process starts with a fast ( 100 fs) planar relaxation from the S2 ππ* state into the πoπ* minimum of the S2 state. The calculations demonstrate that two π-excited states (denoted ππ* and πoπ*) are actually involved in this stage. The time in reaching the S2/S1 intersections, through which thymine can deactivate to S1, is delayed by both the change in character between the states as well as the flatness of the S2 surface. This deactivation occurs in an average time of 2.6 ps at the lowest-energy region of the crossing seam. After that, thymine relaxes to the nπ* minimum of the S1state, where it remains until the transfer to the ground state takes place. The present dynamics simulations show that not only the πoπ* S2 trapping but also the trapping in the nπ* S1 minimum contribute to the elongation of the excitedstate lifetime of thymine. Disciplines Chemistry Comments Reprinted (adapted) with permission from Journal of Physical Chemistry A 113 (2009): 12686, doi: / jp905085x. Copyright 2009 American Chemical Society. Authors Jaroslaw J. Szymczak, Mario Barbatti, Jason T. Soo Hoo, Jaclyn A. Adkins, Theresa Lynn Windus, Dana Nachtigallová, and Hans Lischka This article is available at Iowa State University Digital Repository: 12686 J. Phys. Chem. A 2009, 113, Photodynamics Simulations of Thymine: Relaxation into the First Excited Singlet State Jaroslaw J. Szymczak,*, Mario Barbatti,*, Jason T. Soo Hoo, Jaclyn A. Adkins, Theresa L. Windus, Dana Nachtigallová, # and Hans Lischka*,,# Institute for Theoretical Chemistry, UniVersity of Vienna, Waehrinegrstrasse 17, A 1090 Vienna, Austria, Departments of Physics, Siena College, 515 Loudon Road, LoudonVille, New York 12211, Department of Chemistry, Northwest Missouri State UniVersity, 800 UniVersity DriVe, MaryVille, Missouri 64468, Department of Chemistry, Iowa State UniVersity, 1605 Gilman Hall, Ames, Iowa 50011, and Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, FlemingoVo nam. 2, CZ Prague 6, Czech Republic ReceiVed: May 30, 2009; ReVised Manuscript ReceiVed: August 1, 2009 Ab initio nonadiabatic dynamics simulations are reported for thymine with focus on the S 2 f S 1 deactivation using the state-averaged CASSCF method. Supporting calculations have been performed on vertical excitations, S 1 and S 2 minima, and minima on the crossing seam using the MS-CASPT2, RI-CC2, MR-CIS, and MR- CISD methods. The photodynamical process starts with a fast ( 100 fs) planar relaxation from the S 2 ππ* state into the π O π* minimum of the S 2 state. The calculations demonstrate that two π-excited states (denoted ππ* and π O π*) are actually involved in this stage. The time in reaching the S 2 /S 1 intersections, through which thymine can deactivate to S 1, is delayed by both the change in character between the states as well as the flatness of the S 2 surface. This deactivation occurs in an average time of 2.6 ps at the lowest-energy region of the crossing seam. After that, thymine relaxes to the nπ* minimum of the S 1 state, where it remains until the transfer to the ground state takes place. The present dynamics simulations show that not only the π O π*s 2 trapping but also the trapping in the nπ* S 1 minimum contribute to the elongation of the excitedstate lifetime of thymine. 1. Introduction Upon UV excitation, all five naturally occurring nucleobases return to the ground state on an ultrafast time scale ranging from half a picosecond to a few picoseconds. 1 5 The ultrafast decay minimizes the time that the molecule remains in reactive excited states, which could induce photochemical damage. This enhanced photostability might have been one factor favoring the selection of these bases in early biotic ages, to the detriment of other similar molecules with long-lived excited states. In general, ultrafast decay depends on the existence of reaction pathways connecting the Franck-Condon region to the seam of conical intersections between the excited and ground states where radiationless processes can occur. For this reason, a great deal of theoretical work has been dedicated to the characterization of reaction paths and conical intersections not only for the five nucleobases 6 18 but also for their isomers, 11,19 substituted species, 12,20,21 and base models. 22,23 Further progress has been achieved by means of dynamics simulations, which attempt to describe the excited-state time evolution and the most frequently accessed reaction pathways. However, balancing the computational costs of dynamics simulations lasting for several picoseconds while still maintaining a proper description of multiple electronic excited states and their nonadiabatic couplings still constitutes a major challenge. Despite the difficulties, semiempirical, density functional, 27 and ab initio nonadiabatic dynamics simulations have been reported recently for nucleobases. Part of the Russell M. Pitzer Festschrift. * Corresponding authors. (J.J.S.); (M.B.); (H.L.). University of Vienna. Siena College. Northwest Missouri State University. Iowa State University. # Academy of Sciences of the Czech Republic. Among the five nucleobases, thymine has the longest lifetime. 1,2 Femtosecond-resolved pump-probe resonant ionization experiments pumped at 267 nm have revealed two exponential decay components, 6.4 and 100 ps, with the longer one assigned to the triplet-state population. 2 Another set of massselected femtosecond-resolved pump-probe resonant ionization experiments also pumped at 267 nm and identified a two-step mechanism with time constants 105 fs and 5.12 ps. 1 In the timeresolved photoelectron spectroscopy experiments reported in ref 4, three time constants were obtained, 50, 490 and 6.4 ps (pump energy at 250 nm). Even though there is no full agreement about the details of the deactivation process, a time constant in the range of 5 to 6 ps for deactivation to the ground state clearly emerges from all of these experimental results. This time constant is, in addition, much larger than those measured for the other nucleobases (adenine: 1.1 ps, cytosine: 1.86 ps, guanine: 0.36 ps, uracil: 1.05 ps 1 ). On the basis of the reaction paths connecting the Franck- Condon region to the S 1 /S 0 conical intersections, Perun et al. 13 have proposed that the relatively long lifetime of thymine could be explained by a trapping of the molecule in the dark S 1 nπ* state after fast deactivation from the S 2 ππ* state. Nevertheless, multiple spawning dynamics simulations performed by Hudock et al. 29 at the complete active space self-consistent field (CASSCF) level have found a surprisingly small S 2 f S 1 deactivation yield in the first half picosecond. These authors then proposed that the reason for the long lifetime is the trapping of thymine in a S 2 minimum right after the photoexcitation. On the basis of the analysis of the reaction paths connecting the minimum in the S 1 state to the S 1 /S 0 conical intersections, Zechmann and Barbatti 14 have discussed how the low efficiency of those paths should be an additional factor adding to the S 2 trapping to delay the deactivation to the ground state. Recently /jp905085x CCC: $ American Chemical Society Published on Web 08/19/2009 Photodynamics Simulations of Thymine J. Phys. Chem. A, Vol. 113, No. 45, reported nonadiabatic dynamics simulations for thymine at the OM2 semiempirical level 26 did not show S 2 trapping, and the S 2 f S 1 deactivation was predicted to occur in only 17 fs. The S 1 f S 0 deactivation took place mainly by means of a reaction path involving the nπ* state and occurred in 420 fs, which is one order of magnitude shorter than the experimental results. Merchán et al. 32 have proposed a unified model to explain the ultrafast decay of the pyrimidine nucleobases. According to these authors, the deactivation of thymine, uracil, and cytosine can be explained on the basis of the ππ* state alone, without any relevant influence of the nπ* state. After the excitation into the ππ* state, each of these molecules would either follow a barrierless ππ* path to the conical intersection with the ground state or relax into a S 1 ππ* minimum before finally moving toward the conical intersection with the ground state. These two paths would give origin to two time constants: one in the femtoseconds time scale related to the direct path and another in the picosecond time scale related to the indirect path. In summary, three different hypotheses have been proposed to explain the long lifetime of thymine: (1) trapping in the dark S 1 nπ* state, 13 (2) trapping in the S 2 ππ* state, 29 and (3) trapping in the S 1 ππ* state. 32 In the present work, nonadiabatic dynamics simulations are reported for thymine performed at the CASSCF level propagated for a simulation time of 3 ps, which has been long enough to determine the time constant for the S 2 f S 1 deactivation process and to examine in detail the mechanistic processes. The dynamics simulations show that the S 2 -trapping hypothesis, 29 proposed on the basis of a short time window of 0.5 ps, is fully supported by longer dynamics simulations and that the nπ* state plays an important role in the subsequent steps. 2. Computational Details Mixed quantum-classical dynamics simulations were performed for thymine at the CASSCF level. The active space was composed of ten electrons in eight orbitals (CASSCF(10,8)). At the ground-state minimum geometry, these are composed of one n, four π, and three π* orbitals. (See Figure S1 in the Supporting Information.) State averaging was performed over three states (SA-3), and the 6-31G* 33 basis set was employed. Analytic energy gradients, nonadiabatic coupling vectors, and minima on the crossing seam were computed by the procedures described in refs We performed mixed quantum-classical dynamics 39 by integrating Newton s equations for the nuclear motion in time steps of 0.5 fs using the Velocity-Verlet algorithm 40 and the timedependent electronic Schroedinger equation with the fifth-order Butcher algorithm. 41 The partial coupling approximation 42 was used to reduce the number of nonadiabatic coupling vectors computed in each time step. The time-dependent adiabatic populations were corrected for decoherence effects 43 (R)0.1 hartree) and used for computing the surface hopping probabilities for nonadiabatic transitions according to the fewestswitches algorithm 39,44 in the version proposed by Hammes- Schiffer and Tully. 45 Initial geometries and velocities were generated by a Wigner distribution treating each nuclear coordinate as a harmonic oscillator in the ground state. This distribution is characterized by the absorption spectrum in Figure S2 of the Supporting Information. The absorption spectrum was computed by the Gaussian broadening method described in ref 46. Seventy trajectories were computed with a microcanonical ensemble for at least 1.5 ps. For a subset of 35 trajectories, the simulation time was continued to 3 ps. Thymine structures were analyzed in terms of the Cremer-Pople parameters 47 using the Boeyen s conformer classification scheme. 48 Additional static calculations have been performed with the multireference configuration interaction method including single (MR-CIS) and single and double (MR-CISD) excitations, with the complete active space self-consistent-field second-order perturbation theory in its multistate version (MS-CASPT2) 49 and with the resolution-of-identity approximate coupled cluster to the second-order method (RI-CC2) method. The MR-CISD and MR-CIS calculations were performed with a reference space containing six electrons in five orbitals using the orbitals computed at the CASSCF(10,8)/6-31G* level. We obtained this reference space from the CAS(10,8) space by applying a selection scheme based on natural-orbital occupation numbers where orbitals with an occupation larger than 0.9 and smaller than 0.1 were moved to doubly occupied and virtual space, respectively. Higher-order excitation effects were computed by the Davidson correction 35,53,54 for single-point calculations at the MR-CISD level. For the MS-CASPT2 calculations, we used the same CAS(10,8) space as that before by applying an IPEA shift 55 of 0.25 unless indicated differently. The 6-311G** and the 6-31G* basis sets were used. 33 The RI-CC2 calculations were performed with the TZVPP basis sets. 56 The MRCI calculation were performed with the COLUMBUS program system For the dynamics simulations, the NEW- TON-X program was used. 46,60 RI-CC2 calculations were performed with TURBOMOLE, 61 and MS-CASPT2 computations were performed with the MOLCAS program. 62 The Cremer-Pople parameters were obtained with help of the PLATON program Results and Discussion 3.1. Potential Energy Surface of Thymine. The low-energy UV spectrum of thymine is characterized by a dark singlet S 1 (nπ*) state closely followed by a bright S 2 (ππ*) state. 14,32,64,65 Results obtained in this work are collected in Table 1. The vertical excitation energy into the nπ* state computed with different methods and basis sets agrees within a range of 0.4 ev. The increase in the basis set from double- to triple-ζ quality reduces the excitation energy by 0.2 ev. The effect of the IPEA shift in the MS-CASPT2 calculations is not pronounced for this particular state. As expected, the excitation into the ππ* state is much more sensitive to the method. RI-CC2 excitation energies are reduced by 0.28 ev when the basis set is increased. At the MS-CASPT2 level, this stabilization amounts to only 0.14 ev. The inclusion of the IPEA shift in the MS-CASPT2 calculations increases the ππ* excitation energy by almost 0.4 ev. The MR-CISD+Q excitation energy exceeds the MS- CASPT2 and RI-CC2 results by 0.3 ev. At the CASSCF level used in the dynamics, the ππ* state is found to be too high by about 1.5 to 2 ev. This is a well-known effect of the method 66,67 whose origin is connected to the lack of dynamical electron correlation and of diffuse basis functions. As will be discussed below, other topographic features of the potential energy surfaces computed at CASSCF level are in good agreement with results obtained at higher theoretical levels. The minimum in the S 1 nπ* state optimized at the CASSCF level (Table 1 and Figure 1) shows a planar geometry with an elongation of the C 4 -O 8 and C 5 -C 6 bonds and shortening of the C 4 -C 5 bond in comparison with the ground-state minimum geometry. Cartesian coordinates for this and all other structures discussed in this work are given in the Supporting Information. The S 2 state (Table 1 and Figure 1) is of π O π* character and possesses a minimum with similar elongation of the C 4 -O 8 and 12688 J. Phys. Chem. A, Vol. 113, No. 45, 2009 Szymczak et al. TABLE 1: Vertical Excitation Energies, Energies of the S 1 and S 2 Minima and Energy of the Lowest MXS for Thymine Obtained with Several Methods energy (ev) geometry state CASSCF a CC2 PT2/6-31G* b PT2/6-311G** c MR-CISD d exptl min S 0 S 0 cs e (0.00 f ) 0.00 (0.00) 0.00 (0.00) 0.00 S 1 nπ* (4.87) 5.29 (5.07 g ) 5.23 (5.00 g ) 5.31 S 2 ππ* (5.28) 5.58 (5.22 g ) 5.44 (5.06 g ) h min S 1 S 0 cs (1.48) S 1 nπ* (3.83) S 2 ππ* (5.09) min S 2 S 0 cs S 1 nπ* S 2 π O π* MXS 3,6 B S 0 cs (2.42) S 1 /S 2 ππ*/nπ* (4.85) a SA-3-CASSCF(10,8)/6-31G* (E ref ) au). b MS-CASPT2/SA-3-CAS(10,8)/6-31G* using CASSCF geometries (E ref ) au). c MS-CASPT2/SA-3-CAS(10,8)/6-311G** using CASSCF geometries (E ref ) au). d MR-CISD(6,5)+ Q/SA-3-CASSCF(10,8)/6-31G* (E ref ) au). e RI-CC2/SV(P) using geometries optimized at the same level (E ref ) au). f RI-CC2/TZVPP using geometries optimized at the same level (E ref ) au). g Values obtained with IPEA ) 0. h Ref 64. Figure 1. Ground- and excited-state minima of thymine and valence bond structures based on bond distances. Bond distances are given in angstroms, and main changes are underlined. C 5 -C 6 bonds and shortening of the C 4 -C 5 bond. Different from the S 1 minimum, the S 2 minimum is pyramidalized at C 6. The existence of this minimum has been previously reported on the basis of the CASPT2 single-point and geometry optimizations. 29,68 This finding has been confirmed by optimization at the MR- CISD/6-31G* level performed in this work. The MR-CISD geometry is very similar to the CASSCF geometry. The rootmean-square deviation of the bond distances is smaller than Å, and the maximum deviation occurs for the C 4 C 5 bond distance, which is 0.02 Å shorter at CASSCF than at MR-CISD. No S 2 minimum could be located at the RI-CC2 level. Starting at the ground-state geometry, the ππ* state is connected through a barrierless path to a conical intersection with the ground state, 13,32 creating a direct diabatic pathway for internal conversion. Along the stabilization of the ππ* state, it crosses the nπ* state. The nature of this crossing will be discussed in detail below. For now, it is important to bear in mind that when the crossing occurs there are two relaxation possibilities on the S 1 surface: either continuation with ππ* character or change to nπ* character. In the nπ* state, two kinds of pathways for internal conversion exist: connecting the nπ* minimum to either the nπ*/s 0 or the ππ*/s 0 conical intersections. 14 In the first case, the path shows an uphill profile, whereas in the second case, a barrier needs to be overcome. The crossing between the S 2 and S 1 states occurs predominately at geometries puckered at the C 6 atom. We have identified TABLE 2: Energies of the MXSs for Thymine Relative to the Ground-State Energy Minimum Obtained at the SA-3-CASSCF(10,8)/6-31G* Level MXS a state energy (ev) 3,6 B S S 1 /S E S S 1 /S planar S S 1 /S E 5 S S 1 /S T 1 S S 1 /S S 5 S S 1 /S a B: boat; T: twisted boat; S: screw boat; E: envelope. The superscript and subscript indicate the atoms puckered above and below the ring plane, respectively. six different minima on the S 2 /S 1 intersection seam (MXS). Their energies are collected in Table 2, and their geometric structures are shown in Figure 3. Bond distances and Cremer-Pople parameters are given in Figure S3 and Table S1 of the Supporting Information. With the exception of the planar MXS, which corresponds to a σπ*/ππ* crossing (σ orbital along the N 3 C 4 bond), all other MXSs correspond to ππ*/nπ* crossings. Photodynamics Simulations of Thymine J. Phys. Chem. A, Vol. 113, No. 45, Figure 2. Molecular orbitals involved in the ππ*, nπ*, and π O π* excitations. Figure 3. Geometries of the S 2 /S 1 MXSs. The lowest MXS (see Table 2), which has also been characterized by Hudock et al., 29 shows a boat conformation puckered at atoms N 3 and C 6 ( 3,6 B). The g and h ve
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