UV wavelength-dependent photoionization quantum yields for the dark ¹nπ* state of aqueous thymidine

Abstract

Despite the important role of the dark ¹nπ* state in the photostability of thymidine in aqueous solution, no detailed ultraviolet (UV) wavelength-dependent investigation of the ¹nπ* quantum yield (QY) in aqueous thymidine has been experimentally performed. Here, we investigate the wavelength-dependent photoemission spectra of aqueous thymidine from 266.7 to 240 nm using liquid-microjet photoelectron spectroscopy. Two observed ionization channels are assigned to resonant ionizations from ¹ππ* to the cationic ground state D₀ (π⁻¹) and ¹nπ* to the cationic excited state D₁ (n⁻¹). The weak ¹nπ* → D₁ ionization channel appears due to ultrafast ¹ππ* → ¹nπ* internal conversion within the pulse duration of ∼180 fs. The obtained ¹nπ* quantum yields exhibit a strong wavelength dependence, ranging from 0 to 0.27 ± 0.01, suggesting a hitherto uncharacterized ¹nπ* feature. The corresponding vertical ionization energies (VIEs) of D₀ and D₁ of aqueous thymidine are experimentally determined to be 8.47 ± 0.12 eV and 9.22 ± 0.29 eV, respectively. Our UV wavelength-dependent QYs might indicate that different structural critical points to connect the multidimensional ¹ππ*/¹nπ* conical intersection seam onto the multidimensional potential energy surface of the ¹ππ* state might exist and determine the relaxation processes of aqueous thymidine upon UV excitation.

---

Introduction

Highly efficient electronic relaxation from an optically bright ¹ππ* state to the ground state (S₀) is nowadays regarded as the main photoprotective mechanism for the ultraviolet (UV) photostability of DNA.^1–5 For pyrimidine nucleobases, their photodeactivation mechanisms differ significantly from those of purine nucleobases.^6–11 In addition to directly conically intersecting with the ground state through the non-adiabatic ¹ππ* pathway, pyrimidine nucleobases exhibit an additional relaxation pathway transitioning from the bright ¹ππ* state to the optically dark ¹nπ* in both the theoretical findings^12–18 and the experimental studies.^19–35 The prolonged lifetime of the optically dark ¹nπ* state leads to undesirable photochemical damage,^36–39 and more importantly, the ¹nπ* state can serve as a crucial intermediate in the transition to the ³ππ* state.^40–43 Yet in spite of many ongoing experimental and computing investigations of pyrimidine nucleobases,^{11,17,18,28–36,43} the quantum yield (QY) for the dark ¹nπ* state of thymidine nucleobases, especially in aqueous solution, is very puzzling.^{20,22,28,31,33,35,43} For instance, thymine exhibited an extremely high ¹nπ* QY (∼1.0) in the gas phase compared with in aqueous solution (0.09–0.19), which was reported by extreme ultraviolet time-resolved photoelectron spectroscopy (EUV-TRPES).³⁵ The ¹nπ* QY (0.04–0.06) for aqueous uracil was reported to be considerably lower using EUV-TRPES³⁵ than the previously estimated value (0.28) reported using transient absorption spectroscopy (TAS).²²

The ¹nπ* QY seems to be sensitive to not only the water environment but also the probing techniques.^{8,16,20,22,23,28,31,33,35,43} Compared with TAS^{10,16,22–26,32,33} and fluorescence up conversion (FU),^8,9,20,21 photoionization is the most efficient way to detect a dark state when implemented with photoelectron spectroscopy (PES).^28,31,35,43 However, the ¹nπ* state of aqueous thymine and thymidine was not detected by ultraviolet time-resolved photoelectron spectroscopy (UV-TRPES).^28,31 Suzuki and co-workers have recently identified the existence of the dark ¹nπ* state and reasonably resolved the contradiction between UV-TRPES^28,31 and EUV-TRPES³⁵ results. Despite these advances,^6–42 no detailed wavelength-dependent investigation of ¹nπ* QY in aqueous pyrimidine nucleobases has been so far experimentally performed. The wavelength-dependent ¹nπ* QY in aqueous pyrimidine nucleobases is crucial to understanding the DNA damage,^1,4,33,35e.g., the formation of the long-lived ³ππ* state,^{26,29,33,35,43} unwanted cyclobutene pyrimidine dimers or 6-4 photoproducts.^1,4,21,22,37

Herein, we investigate the wavelength-dependent photoemission spectra of aqueous thymidine from 266.7 to 240 nm using liquid-microjet PES in combination with the resonantly enhanced multiphoton ionization (REMPI) scheme. A feature of the femtosecond REMPI-PES is that it provides a dynamical probe of photoionization from both optically prepared excited states and secondarily populated dark states within the pulse duration, as far as the lifetimes of these excited states are appropriate. Moreover, REMPI-PES offers an easier way to separate the resonant signals from the nonresonant ones by varying the ionization wavelength, which helps to identify the origin of the ionization channels. Notably, the wavelength-dependent ¹nπ* QYs for aqueous thymidine are experimentally determined for the first time.

Results and discussion

Thymidine (Thd) with high solubility in water was chosen as its ¹nπ* QY (0.05) in aqueous solution previously reported by TAS³³ seems to behave rather differently from that (0.18–0.22) recently reported by EUV-TRPES.³⁵ As shown in Fig. 1, the measured UV absorption spectrum of aqueous Thd exhibits two strong absorption features at ∼267 nm and ∼205 nm. The lower energy absorption band at ∼267 nm arises from a first ππ* transition (denoted as ¹ππ*) from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), as shown in Fig. 2, while the higher energy absorption band involves multiple ππ* transition contributions.⁴ The optically dark ¹nπ* state is lower in energy than the ¹ππ* state^31,35 and is strongly destabilized in water due to a reduction in solute–water interactions.²² Despite different interpretations of the relaxation of the ¹ππ* state, the recent EUV-TRPES results³⁵ reported by Suzuki and co-workers concluded that the ¹ππ* state internally converts to both S₀ and ¹nπ* states following UV excitation. The ¹ππ* → ¹nπ* internal conversion (IC) occurs within an ultrashort time of ∼87 fs.³⁵ However, the ¹nπ* state seems to extend as extremely broad to the higher-energy absorption side of the ¹ππ* state by theoretical insights.^31,35 This led us to undertake eight wavelengths for ionization in the range 266.7–240 nm, i.e., spanning from the maximum to the blue-sided absorption of the ¹ππ* state. We employ one-color (1 + 2) REMPI-PES to measure the photoelectron kinetic energy (PKE) distribution and then determine the electron binding energy (eBE) of the molecular orbital from which the electron is ionized by eBE = 3hν − PKE, where hν represents the photon energy and eBE refers to the binding energy from the S₀ state to the ionic state.

Fig. 1 UV absorption spectrum of 10 μM aqueous thymidine (blue shaded area). Eight vertically colored lines locate the ionization wavelengths utilized in our (1 + 2) UV PES measurements, spanning from the maximum (266.7 nm) to the blue-sided absorption (240.0 nm) of the ¹ππ* state. Inset: Ground-state minimum energy geometry structure (carbon, oxygen, nitrogen and hydrogen atoms shown in grey, red, blue and white, respectively) of thymidine + 6H₂O calculated using the B3LYP/6-311G(d,p) method.

Fig. 2 Schematic UV photoionization mechanism of aqueous thymidine illustrating the resonantly enhanced 1 + 2 photoionization process. The first photon prepares the optically bright ¹ππ* state, and then ultrafast ¹ππ* → ¹nπ* internal conversion (IC) within the pulse duration of ∼180 fs populates the low-lying dark ¹nπ* state. The ensuing two-photon absorption preferentially ionizes the ¹ππ* and ¹nπ* states into D₀ (π⁻¹) and D₁ (n⁻¹), respectively. The π (HOMO), n_O (Rydberg orbital) and π* (LUMO) orbitals that result in ¹ππ* and ¹nπ* states are calculated based on a cluster model of thymidine + 6H₂O at the B3LYP/6-311G (d, p) level. The oscillator intensities for the ¹ππ* and ¹nπ* states from the ground state are shown as f₁ and f₂, indicating the ¹nπ* state is nearly dark. See the text for details.

Fig. 3 shows the eBE spectra of 100 mM aqueous Thd with an addition of 30 mM NaF using eight wavelengths for resonant ionization with the S₀–¹ππ* transition with increasing excess energy in ¹ππ*. These spectra feature a broadband within 6.5–10.5 eV, representing a typical feature of an aqueous PES.^{28,31,33,35,43} Moreover, a spectral signature resulting from the ¹b₁ orbital of liquid water can be well observed at a slightly larger energy for each ionization wavelength. As an internal energy standard,^44–48 the accurate eBE value of 11.33 eV for the ¹b₁ orbital of liquid water was used to calibrate the absolute eBE spectrum for each ionization wavelength.^35,46 Therefore, these eBE spectra shown in Fig. 3 are nearly free from the combined influence of our experimental conditions. From careful examination, the line shape of the broadband within 6.5–10.5 eV in each eBE spectrum changes with ionization wavelengths and their energies. All the line shapes at different wavelengths can be well fitted as a superposition of two Gaussian functions with a more intense eBE component centered at ∼8.5 eV and a smaller eBE component centered at ∼9.5 eV, as indicated by the two color-filled Gaussian components shown in Fig. 3. Notably, the central peak positions for the two eBE components vary slightly with ionization wavelengths, and the line shape at 240 nm can be well-fitted by only one intense eBE component centered at ∼8.5 eV. On the basis of a significant body of previous work on aqueous Thd,^{28,31,33,35,49} we assign the eBE component centered at ∼8.5 eV to the cationic ground state D₀ (π⁻¹) and the eBE component centered at ∼9.5 eV to the cationic first excited state D₁ (n⁻¹), which corresponds to the two resonant ionization channels from ¹ππ* → D₀ (π⁻¹) and ¹nπ* → D₁ (n⁻¹), respectively.

Fig. 3 Electron binding energy (eBE) spectra of 100 mM aqueous thymidine with the addition of 30 mM NaF (ESI† S2) obtained by resonantly enhanced 1 + 2 photoelectron kinetic energy (PKE) spectra at eight typical ionization wavelengths from 266.7 nm to 240 nm. The eBE = 3hν − PKE, where hν represents the photon energy, corresponding to the eBE of the cationic states. The black dots represent experimental data, and the red line represents the sum of the Gaussian component fits. The light green shaded Gaussian component centered at ∼8.5 eV and the dark blue shaded Gaussian component centered at ∼9.5 eV correspond to the two resonant ionization channels from ¹ππ* → D₀ (π⁻¹) and ¹nπ* → D₁ (n⁻¹), respectively. The vertically dashed line indicates the vertical ionization energy value of liquid water 1b₁. See the text for details. Note that the fitting function profile with a Gaussian or Voigt function might affect the spectral fitting results for the ¹nπ*quantum yields but to an acceptable extent (see the ESI† for details).

We ascertained this assignment by an analysis of the following aspects. Firstly, the assignment of the two resonant ionization channels is based on the Koopmans’ theorem and Frank–Condon principle.⁵⁰ We calculated the Dyson norms^35,51 to examine Koopmans’ ionization correlations for a cluster model^20,52 of Thd with six water molecules (ESI† S6): ¹ππ* preferentially ionizes into D₀ (π⁻¹), whereas ¹nπ* preferentially ionizes into D₁ (n⁻¹). As shown in the ESI† S6, the VIE and adiabatic ionization energy (AIE) of ¹ππ* are calculated to be 8.40 and 7.97 eV, respectively. The VIE − AIE difference (0.43 eV) is the vibrational energy left in D₀ following photoionization from the ¹ππ*. Likewise, the vertical excitation energy (VEE) and adiabatic excitation energy (AEE) of ¹ππ* are calculated to be 5.07 and 4.96 eV, respectively. The VEE − AEE difference (0.11 eV) is close to the VIE − AIE difference, consistent with the fact that the rigid aromatic systems conserve vibrational energy during photoionization.⁵⁰ The observed vertical eBEs of ∼8.5 eV (together with our calculated value of ∼8.40 eV) and ∼9.5 eV can be directly compared to the corresponding vertical eBEs D₀ = 8.3,⁴⁹ ∼8.5 eV,³⁵ D₁ = ∼9.7,⁴⁹ ∼9.8³⁵ eV known or estimated from previous work,^35,49 confirming in part this assignment. Secondly, the ionization propensity rule allows us to use the vibrational Frank–Condon envelope to identify resonant and nonresonant (1 + 2) ionization processes.⁵¹ The nonresonant (1 + 2) ionization from S₀ results in a constant eBE vibrational envelope, whereas resonant (1 + 2) ionization from ¹ππ* or ¹nπ* results in an energy-shifted eBE vibrational envelope. The two observed eBE components are found to shift around ∼8.5 eV and ∼9.5 eV with different ionization wavelengths (ESI,† S5), reflecting the actual wavelength resonance with the vibrionic structure of the ¹ππ* or ¹nπ* upon the first photon absorption.

Thirdly, it is noteworthy that ¹nπ* is not expected to be excited directly although its calculated oscillator strength (f₁ = 0.0059) from S₀ which is not exactly equal to zero. Thus, the observation of the ¹nπ* → D₁ (n⁻¹) resonant ionization channel suggests that the ¹nπ* state is likely to be populated by ultrafast IC from the ¹ππ* state, which is excited directly, within the pulse duration of ∼180 fs. Due to the high time resolution of sub-50 fs, the recent EUV-TRPES result of 150 mM aqueous Thd revealed the mostly exact time of 87 fs for ¹ππ* → ¹nπ* IC.³⁵ Together with earlier time constants of 120 fs²⁸ and 100 fs³³ for the ¹ππ* state to relax on its surface, it immediately indicates that the ¹ππ* → ¹nπ* IC can occur within the pulse duration of ∼180 fs. As seen in Fig. 3, a bifurcation can be experimentally deduced from the ¹ππ* signal with the majority proposed to be directly relaxing to S₀, with the remainder internally converting to ¹nπ*. Moreover, the IC time of 250–390 fs^28,31,35 for ¹ππ* → S₀ is much longer than that for ¹ππ* → ¹nπ*,³⁵ whilst the lifetime for the ¹nπ* is as long as multiple picoseconds.³⁵ These time differences make the observation of the two ionization channels within the pulse duration possible. Note that ultrafast IC or other nonadiabatic processes occurring within the pulse duration are not without precedent.⁵¹ Fourthly, to distinguish between the two- and three-photon ionization schemes, we further examine the laser power dependence of the ionizing liquid water ¹b₁ signal. Other unidentified stronger signals overlap with the above two ionization channels when the laser energy increases to more than 1000 nJ, resulting in a big uncertainty in extracting the two ionization channel components. As illustrated in the ESI,† S3, however, the ¹b₁ signal shows no interruption and almost linear laser power dependence. The slope of the straight line is ∼2.4, indicative of a preferred three-photon ionization scheme. These aspects, taken together, reasonably support our assignment.

We now focus on wavelength dependences of the eBE spectra shown in Fig. 3. Experimentally, the minimum laser energy required for three-photon ionization near 266.7 nm is about 40–50 nJ. As the wavelength decreases, the observation of the aqueous Thd signal becomes possible only when the laser energy is continuously increased to the appearance threshold, as shown in Fig. 3. This is certainly an interesting experimental finding, well consistent with the UV absorption spectrum of aqueous Thd shown in Fig. 1. As the wavelength decreases from 266.7 nm, the ¹ππ* absorption gradually decreases from its maximum to its blue-sided weak region. Thus, more laser energy is required for three-photon ionization as the wavelength decreases. In particular, the spectral line shape changes with ionization wavelength and laser energy, as seen from Fig. 3. At 266.7 nm, the eBE spectrum can be well fitted by two Gaussian functions with a strong ¹ππ* → D₀ (π⁻¹) component centered at ∼8.5 eV and a smaller ¹nπ* → D₁ (n⁻¹) component centered at ∼9.5 eV. This feature is also obviously identifiable by eyes at 260.5, 254.8, 246.3 and 243.2 nm, but not clear at 258.0 and 256.2 nm. A detailed comparison between the two-Gaussian and one-Gaussian fits at 258.0 and 256.2 nm indicates that the two-Gaussian model can give the best acceptable fit (ESI,† S4). In contrast, the one-Gaussian model gives the best acceptable fit at 240 nm, revealing only the ¹ππ* → D₀ (π⁻¹) component centered at ∼8.5 eV. When the laser energy is increased to more than 550 nJ, the ¹nπ* → D₁ (n⁻¹) component centered at ∼9.5 eV still disappears at 240 nm. Summarizing these features, a wavelength-dependent photoionization picture can be described as follows. As shown in Fig. 2, during the resonant (1 + 2) ionization process, the first photon resonantly prepares the optically bright ¹ππ* state, and then ultrafast (within the pulse duration of ∼180 fs) IC populates the low-lying dark ¹nπ* state. The ensuing two-photon absorption preferentially ionizes the ¹ππ* and ¹nπ* states into D₀ (π⁻¹) and D₁ (n⁻¹), respectively. The two ionization channels exhibit some dependences on ionization wavelength and laser energy. As the wavelength decreases from 266.7 to 240 nm, the two ionization channels show energy-shifted eBE vibrational envelopes around ∼8.5 eV and ∼9.5 eV, whilst more laser energy is required for the three-photon ionization since the ¹ππ* absorption cross section gradually decreases from its maximum to its blue-sided weak region. Specifically, the ¹nπ* → D₁ (n⁻¹) ionization channel disappears at 240 nm.

Of particular interest is the wavelength-dependent QY for the ¹nπ*state, which can be obtained from the integrated area intensity ratio of I[¹nπ*]/(I[¹nπ*] + I[¹ππ*]) at different ionization wavelengths. As shown in Fig. 4, the QYs indicate the ¹nπ* → D₁ (n⁻¹) ionization channel as the minor component exhibits strong dependences on ionization wavelength. At 266.7 nm, the QY has the highest value of 0.27. Then, the QYs continue to decrease until the wavelength decreases to 256.2 nm. At this particular wavelength, the QY has the lowest value of 0.02. This feature of the QYs is consistent with the characteristic of the ¹ππ* absorption cross-section between 266.7 and 256.2 nm, as discussed earlier. Unexpectedly, the QYs begin to increase as the wavelength continues to decrease from 256.2 nm. At 243.2 nm, the QY reaches the highest value of 0.17. However, the QY becomes zero at 240 nm. The QYs between 256.2 and 243.2 nm give a relationship contrary to expectation since the ¹ππ* absorption cross-section gradually decreases from 266.7 to 240 nm. At 258.0 nm, the QY is 0.08, which is much smaller than the EUV-TRPES value³⁵ of 0.18–0.22 obtained at 257 nm excitation, but is somewhat surprisingly consistent with the TAS value³³ of 0.05 obtained at 275.5 nm excitation (roughly equivalent to 258.0 nm as their nearly equal absorption cross sections). The large variation of the QYs with wavelengths may be attributed to the fact that different vibrational excitations in the ¹ππ* state correspond to different energetic and structural information about their nuclear configurations in multidimensional coordinates, which determine the different dynamic critical points to connect the multidimensional ¹ππ*/¹nπ* conical intersection (CI) seam on multidimensional potential energy surfaces.^15,28,33,41 In particular, it might be impossible to access the ¹ππ*/¹nπ* CI following 240 nm or shorter wavelength excitation as its nuclear configuration may be extraordinarily far away from the ¹ππ*/¹nπ* CI. Despite no signature of ¹nπ* in the UV-TRPES results,^28,31 the pump wavelength dependence gives a contrary expectation of the increasing ¹ππ* lifetime as the pump wavelength decreases from 261 to 240 nm. Therefore, to search for possible critical points that connect the multidimensional CI seam in this system may be intriguing, but certainly high-level theoretical calculations are desirable in the future.

Fig. 4 The quantum yields (QYs) for the ¹nπ* state at eight typical ionization wavelengths from 266.7 nm to 240 nm. The QYs were obtained from the integrated area intensity ratio of I[¹nπ*]/(I[¹nπ*] + I[¹ππ*]) at each ionization wavelength, where the integrated area intensities of I[¹nπ*] and I[¹ππ*] were extracted by the two Gaussian component fits shown in Fig. 3. See the text for details of the illustration of wavelength-dependent QYs.

Lastly, several aspects are worthy of notice. First, note that the acidity or alkalinity of solvents could impact the charge distribution and transition rates of excited-state molecules by altering their charge states through protonation or deprotonation processes.¹⁹ However, we did not consider this influence in our work. Furthermore, whether the addition of the halide salts may affect the electronic structure (i.e., energies, energy order, multiphoton absorption or ionization cross sections of the excited states) of thymidine and its excited state dynamics is currently unknown. Our steady-state absorption experiments seemingly indicate that the addition of NaF does not significantly affect the UV absorption of aqueous thymidine under UV irradiation ranging from 240.0 nm to 266.7 nm. Moreover, a comparison of the features of the first and second absorption bands of the aqueous thymidine solutions with and without the irradiated UV laser pulses seems to suggest that there is a negligible influence on the first absorption bands by the thymidine dimerization and its photodegradation (see the ESI† for details). What is more, according to femtosecond time-resolved infrared experiments on thymine single strands, cyclobutane thymine dimers are fully formed in ∼1 picosecond after ultraviolet excitation.³⁸ More importantly, note that the concentration for the thymidine monomer is usually at least an order of magnitude larger than that for the thymidine dimers in the aqueous thymidine solution of high concentration. These above aspects might also explain why Prof. Suzuki's group claimed that the lifetimes and quantum yield for thymidine are not strongly influenced by any aggregation of thymidine nucleobases.³⁵ Therefore, we speculate that the thymidine photodegradation might not occur during our photoexcitation and photoionization processes within a time window of ∼180 fs. As expected, future work is desirable concerning with how to assess the concentration effect on thymidine dimerization and its photodegradation, the influence of the addition of halide salts on the electronic structure and the excited state dynamics of aqueous thymidine.

Conclusions

We measured the wavelength-dependent eBE spectra of aqueous thymidine from 266.7 to 240 nm. Two resonant ionization channels from ¹ππ* → D₀ (π⁻¹) and ¹nπ* → D₁ (n⁻¹) were observed following the (1 + 2) ionization process. The weak ¹nπ* → D₁ ionization channel appears due to ultrafast ¹ππ*/¹nπ* internal conversion within the pulse duration of ∼180 fs, but disappears at 240 nm. The corresponding ¹nπ* quantum yields exhibit a strong wavelength dependence, ranging from 0 to 0.27 ± 0.01. The corresponding averaged vertical ionization energies (VIEs) of D₀ and D₁ of aqueous thymidine are experimentally determined to be 8.47 ± 0.12 eV and 9.22 ± 0.29 eV, respectively. Our study first unravels a hitherto uncharacterized ¹nπ* feature and provides new insights into understanding aqueous thymidine's photostability. Further high-level theoretical calculations are desirable to elucidate how wavelength-dependent excitation is related to dynamic critical points that connect the multidimensional ¹ππ*/¹nπ* conical intersection seam on multidimensional potential energy surfaces.

Methods

Experiments were conducted by seeding the sample in an HPLC pump and expanding it into a molecular liquid beam apparatus consisting of a magnetic-bottle photoelectron spectrometer and a laser system, as described in detail in the ESI.†

Author contributions

J. L., S. Z. and B. Z. conceived and designed the experiments. P. X., D. W. and J. L. conducted experiments. P. X., D. W., D. L., J. L., S. Z. and B. Z. analyzed the experimental data. P. X. and D. L. performed theoretical calculations. P. X. and J. L. wrote the manuscript. All authors discussed the results and modified the manuscript.

Data availability

The datasets supporting this article have been uploaded as part of the ESI.†

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (No. 2019YFA0307700), the National Natural Science Foundation of China (No. 12274418, 22273116, 12074389, 11974381) and the Knowledge Innovation Program of Wuhan-Basic Research (No. 2022010801010134, 2023020201010084).

Notes and references

1. C. E. Crespo-Hernández, B. Cohen, P. M. Hare and B. Kohler, Ultrafast Excited-State Dynamics in Nucleic Acids, Chem. Rev., 2004, 104, 1977–2019.
2. C. T. Middleton, K. de La Harpe, C. Su, Y. K. Law, C. E. Crespo-Hernández and B. Kohler, DNA Excited-State Dynamics: From Single Bases to the Double Helix, Annu. Rev. Phys. Chem., 2009, 60, 217–239.
3. K. Kleinermanns, D. Nachtigallová and M. S. de Vries, Excited State Dynamics of DNA Bases, Int. Rev. Phys. Chem., 2013, 32, 308–342.
4. R. Improta, F. Santoro and L. Blancafort, Quantum Mechanical Studies on the Photophysics and the Photochemistry of Nucleic Acids and Nucleobases, Chem. Rev., 2016, 116, 3540–3593.
5. V. G. Stavros and J. R. Verlet, Gas-Phase Femtosecond Particle Spectroscopy: A Bottom-Up Approach to Nucleotide Dynamics, Annu. Rev. Phys. Chem., 2016, 67, 211–232.
6. H. Kang, K. T. Lee, B. Jung, Y. J. Ko and S. K. Kim, Intrinsic Lifetimes of the Excited State of DNA and RNA Bases, J. Am. Chem. Soc., 2002, 124, 12958–12959.
7. M. Barbatti, A. J. Aquino, J. J. Szymczak, D. Nachtigallova, P. Hobza and H. Lischka, Relaxation Mechanisms of UV-Photoexcited DNA and RNA Nucleobases, Proc. Natl. Acad. Sci. U. S. A., 2010, 107, 21453–21458.
8. J. Peon and A. H. Zewail, DNA/RNA Nucleotides and Nucleosides: Direct Measurement of Excited-State Lifetimes by Femtosecond Fluorescence Up-Conversion, Chem. Phys. Lett., 2001, 348, 255–262.
9. J. M. Pecourt, J. Peon and B. Kohler, DNA Excited-State Dynamics: Ultrafast Internal Conversion and Vibrational Cooling in a Series of Nucleosides, J. Am. Chem. Soc., 2001, 123, 10370–10378.
10. V. I. Prokhorenko, A. Picchiotti, M. Pola, A. G. Dijkstra and R. J. Miller, New Insights into the Photophysics of DNA Nucleobases, J. Phys. Chem. Lett., 2016, 7, 4445–4450.
11. S. Matsika, Electronic Structure Methods for the Description of Nonadiabatic Effects and Conical Intersections, Chem. Rev., 2021, 121, 9407–9449.
12. H. R. Hudock, B. G. Levine, A. L. Thompson, H. Satzger, D. Townsend, N. Gador, S. Ullrich, A. Stolow and T. J. Martinez, Ab Initio Molecular Dynamics and Time-Resolved Photoelectron Spectroscopy of Electronically Excited Uracil and Thymine, J. Phys. Chem. A, 2007, 111, 8500–8508.
13. J. J. Szymczak, M. Barbatti, J. T. S. Hoo, J. A. Adkins, T. L. Windus, D. Nachtigallová and H. Lischka, Photodynamics Simulations of Thymine: Relaxation into the First Excited Singlet State, J. Phys. Chem. A, 2009, 113, 12686–12693.
14. Z. Lan, E. Fabiano and W. Thiel, Photoinduced Nonadiabatic Dynamics of Pyrimidine Nucleobases: On-the-Fly Surface-Hopping Study with Semiempirical Methods, J. Phys. Chem. B, 2009, 113, 3548–3555.
15. D. Picconi, A. Lami and F. Santoro, Hierarchical Transformation of Hamiltonians with Linear and Quadratic Couplings for Nonadiabatic Quantum Dynamics: Application to the ππ*/nπ* Internal Conversion in Thymine, J. Chem. Phys., 2012, 136, 244104.
16. R. Improta, V. Barone, A. Lami and F. Santoro, Quantum Dynamics of the Ultrafast ππ*/nπ* Population Transfer in Uracil and 5-Fluoro-Uracil in Water and Acetonitrile, J. Phys. Chem. B, 2009, 113, 14491–14503.
17. S. M. Parker, S. Roy and F. Furche, Multistate Hybrid Time Dependent Density Functional Theory with Surface Hopping Accurately Captures Ultrafast Thymine Photodeactivation, Phys. Chem. Chem. Phys., 2019, 21, 18999–19010.
18. W. Park, S. Lee, M. Huix-Rotllant, M. Filatov and C. H. Choi, Impact of the Dynamic Electron Correlation on the Unusually Long Excited-State Lifetime of Thymine, J. Phys. Chem. Lett., 2021, 12, 4339–4346.
19. P. M. Hare, C. E. Crespo-Hernández and B. Kohler, Solvent-Dependent Photophysics of 1-Cyclohexyluracil: Ultrafast Branching in the Initial Bright State Leads Nonradiatively to the Electronic Ground State and a Long-Lived ¹nπ* State, J. Phys. Chem. B, 2006, 110, 18641–18650.
20. T. Gustavsson, A. Banyasz, E. Lazzarotto, D. Markovitsi, G. Scalmani, M. J. Frisch, V. Barone and R. Improta, Singlet Excited-State Behavior of Uracil and Thymine in Aqueous Solution: A Combined Experimental and Computational Study of 11 Uracil Derivatives, J. Am. Chem. Soc., 2006, 128, 607–619.
21. C. Ma, C. C. Cheng, C. T. Chan, R. C. Chan and W. M. Kwok, Remarkable Effects of Solvent and Substitution on the Photo-Dynamics of Cytosine: A Femtosecond Broadband Time Resolved Fluorescence and Transient Absorption Study, Phys. Chem. Chem. Phys., 2015, 17(29), 19045–19057.
22. P. M. Hare, C. E. Crespo-Hernandez and B. Kohler, Internal Conversion to the Electronic Ground State Occurs via Two Distinct Pathways for Pyrimidine Bases in Aqueous Solution, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 435–440.
23. T. Gustavsson, Á. Bányász, N. Sarkar, D. Markovitsi and R. Improta, Assessing Solvent Effects on the Singlet Excited State Lifetime of Uracil Derivatives: A Femtosecond Fluorescence Upconversion Study in Alcohols and D₂O, Chem. Phys., 2008, 350, 186–192.
24. B. Xue, A. Yabushita and T. Kobayashi, Ultrafast Dynamics of Uracil and Thymine Studied Using a Sub-10 fs Deep Ultraviolet Laser, Phys. Chem. Chem. Phys., 2016, 18, 17044–17053.
25. T. J. A. Wolf, R. H. Myhre, J. P. Cryan, S. Coriani, R. J. Squibb, A. Battistoni, N. Berrah, C. Bostedt, P. Bucksbaum, G. Coslovich, R. Feifel, K. J. Gaffney, J. Grilj, T. J. Martinez, S. Miyabe, S. P. Moeller, M. Mucke, A. Natan, R. Obaid, T. Osipov, O. Plekan, S. Wang, H. Koch and M. Gühr, Probing Ultrafast ππ*/nπ* Internal Conversion in Organic Chromophores via K-Edge Resonant Absorption, Nat. Commun., 2017, 8, 29.
26. B. M. Pilles, B. Maerz, J. Chen, D. B. Bucher, P. Gilch, B. Kohler, W. Zinth, B. P. Fingerhut and W. J. Schreier, Decay Pathways of Thymine Revisited, J. Phys. Chem. A, 2018, 122, 4819–4828.
27. S. C. Wei, J. W. Ho, H. C. Yen, H. Q. Shi, L. H. Cheng, C. N. Weng, W. K. Chou, C. C. Chiu and P. Y. Cheng, Ultrafast Excited-State Dynamics of Hydrogen-Bonded Cytosine Microsolvated Clusters with Protic and Aprotic Polar Solvents, J. Phys. Chem. A, 2018, 122, 9412–9425.
28. F. Buchner, A. Nakayama, S. Yamazaki, H. H. Ritze and A. Lubcke, Excited-State Relaxation of Hydrated Thymine and Thymidine Measured by Liquid-Jet Photoelectron Spectroscopy: Experiment and Simulation, J. Am. Chem. Soc., 2015, 137, 2931–2938.
29. A. J. Pepino, J. Segarra-Marti, A. Nenov, I. Rivalta, R. Improta and M. Garavelli, UV-Induced Long-Lived Decays in Solvated Pyrimidine Nucleosides Resolved at the MS-CASPT2/MM Level, Phys. Chem. Chem. Phys., 2018, 20, 6877–6890.
30. T. J. A. Wolf, R. M. Parrish, R. H. Myhre, T. J. Martinez, H. Koch and M. Guhr, Observation of Ultrafast Intersystem Crossing in Thymine by Extreme Ultraviolet Time-Resolved Photoelectron Spectroscopy, J. Phys. Chem. A, 2019, 123, 6897–6903.
31. B. A. Erickson, Z. N. Heim, E. Pieri, E. Liu, T. J. Martinez and D. M. Neumark, Relaxation Dynamics of Hydrated Thymine, Thymidine, and Thymidine Monophosphate Probed by Liquid Jet Time-Resolved Photoelectron Spectroscopy, J. Phys. Chem. A, 2019, 123, 10676–10684.
32. W. Hua, S. Mukamel and Y. Luo, Transient X-ray Absorption Spectral Fingerprints of the S₁ Dark State in Uracil, J. Phys. Chem. Lett., 2019, 10, 7172–7178.
33. R. Borrego-Varillas, A. Nenov, P. Kabacinski, I. Conti, L. Ganzer, A. Oriana, V. K. Jaiswal, I. Delfino, O. Weingart, C. Manzoni, I. Rivalta, M. Garavelli and C. Cerullo, Tracking Excited State Decay Mechanisms of Pyrimidine Nucleosides in Real Time, Nat. Commun., 2021, 12, 7285.
34. P. Chakraborty, Y. Liu, S. McClung, T. Weinacht and S. Matsika, Time Resolved Photoelectron Spectroscopy as a Test of Electronic Structure and Nonadiabatic Dynamics, J. Phys. Chem. Lett., 2021, 12, 5099–5104.
35. Y. Miura, Y. I. Yamamoto, S. Karashima, N. Orimo, A. Hara, K. Fukuoka, T. Ishiyama and T. Suzuki, Formation of Long-Lived Dark States during Electronic Relaxation of Pyrimidine Nucleobases Studied Using Extreme Ultraviolet Time-Resolved Photoelectron Spectroscopy, J. Am. Chem. Soc., 2023, 145, 3369–3381.
36. W. Park, M. Filatov Gulak, S. Sadiq, I. Gerasimov, S. Lee, T. Joo and C. H. Choi, A Plausible Mechanism of Uracil Photohydration Involves an Unusual Intermediate, J. Phys. Chem. Lett., 2022, 13, 7072–7080.
37. B. Milovanovic, M. Kojic, M. Petkovic and M. Etinski, New Insight into Uracil Stacking in Water from ab Initio Molecular Dynamics, J. Chem. Theory Comput., 2018, 14, 2621–2632.
38. W. J. Schreier, T. E. Schrader, F. O. Koller, P. Gilch, C. E. Crespo-Hernández, V. N. Swaminathan, T. Carell, W. Zinth and B. Kohler, Thymine Dimerization in DNA Is an Ultrafast Photoreaction, Science, 2007, 315, 625–629.
39. W. M. Kwok, C. Ma and D. L. Phillips, A Doorway State Leads to Photostability or Triplet Photodamage in Thymine DNA, J. Am. Chem. Soc., 2008, 130, 5131–5139.
40. M. M. Brister and C. E. Crespo-Hernández, Excited-State Dynamics in the RNA Nucleotide Uridine 5′-Monophosphate Investigated Using Femtosecond Broadband Transient Absorption Spectroscopy, J. Phys. Chem. Lett., 2019, 10, 2156–2161.
41. J. Gonzalez-Vazquez and L. Gonzalez, A Time-Dependent Picture of the Ultrafast Deactivation of Keto-Cytosine Including Three-State Conical Intersections, ChemPhysChem, 2010, 11, 3617–3624.
42. M. M. Brister and C. E. Crespo-Hernandez, Direct Observation of Triplet-State Population Dynamics in the RNA Uracil Derivative 1-Cyclohexyluracil, J. Phys. Chem. Lett., 2015, 6, 4404–4409.
43. N. Orimo, Y. I. Yamamoto, S. Karashima, A. Boyer and T. Suzuki, Ultrafast Electronic Relaxation in 6-Methyluracil and 5-Fluorouracil in Isolated and Aqueous Conditions: Substituent and Solvent Effects, J. Phys. Chem. Lett., 2023, 14, 2758–2763.
44. N. Kurahashi, S. Karashima, Y. Tang, T. Horio, B. Abulimiti, Y.-I. Suzuki, Y. Ogi, M. Oura and T. Suzuki, Photoelectron Spectroscopy of Aqueous Solutions: Streaming Potentials of NaX (X = Cl, Br, and I) Solutions and Electron Binding Energies of Liquid Water and X⁻, J. Chem. Phys., 2014, 140, 174506.
45. C. W. West, J. Nishitani, C. Higashimura and T. Suzuki, Extreme Ultraviolet Time-Resolved Photoelectron Spectroscopy of Aqueous Aniline Solution: Enhanced Surface Concentration and Pump-Induced Space Charge Effect, Mol. Phys., 2020, 119, e1748240.
46. S. Thürmer, S. Malerz, F. Trinter, U. Hergenhahn, C. Lee, D. M. Neumark, G. Meijer, B. Winter and I. Wilkinson, Accurate Vertical Ionization Energy and Work Function Determinations of Liquid Water and Aqueous Solutions, Chem. Sci., 2021, 12, 10558–10582.
47. R. Signorell and B. Winter, Photoionization of the Aqueous Phase: Clusters, Droplets and Liquid Jets, Phys. Chem. Chem. Phys., 2022, 24, 13438–13460.
48. M. S. Scholz, W. G. Fortune, O. Tau and H. H. Fielding, Accurate Vertical Ionization Energy of Water and Retrieval of True Ultraviolet Photoelectron Spectra of Aqueous Solutions, J. Phys. Chem. Lett., 2022, 13, 6889–6895.
49. P. Slavíček, B. Winter, M. Faubel, S. E. Bradforth and P. Jungwirth, Ionization Energies of Aqueous Nucleic Acids: Photoelectron Spectroscopy of Pyrimidine Nucleosides and ab Initio Calculations, J. Am. Chem. Soc., 2009, 131, 6460–6467.
50. A. Stolow, A. E. Bragg and D. M. Neumark, Femtosecond Time-Resolved Photoelectron Spectroscopy, Chem. Rev., 2004, 104, 1719–1757.
51. J. W. Riley, B. Wang, J. L. Woodhouse, M. Assmann, G. A. Worth and H. H. Fielding, Unravelling the Role of an Aqueous Environment on the Electronic Structure and Ionization of Phenol Using Photoelectron Spectroscopy, J. Phys. Chem. Lett., 2018, 9, 678–682.
52. M. Etinski and C. M. Marian, Ab Initio Investigation of the Methylation and Hydration Effects on the Electronic Spectra of Uracil and Thymine, Phys. Chem. Chem. Phys., 2010, 12, 4897–4898.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp02594f

---