Article pubs.acs.org/JPCA
Ultrafast Excited-State Dynamics of 6‑Azauracil Studied by Femtosecond Transient Absorption Spectroscopy XinZhong Hua, LinQiang Hua, and XiaoJun Liu* State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China S Supporting Information *
ABSTRACT: The excited-state dynamics of 6-azauracil in different solvents have been studied using femtosecond transient absorption spectroscopy. The molecule is populated to the S2 state with a pump pulse at 264 nm. Broad-band white light continuum which covers from 320 to 600 nm is used as the probe. With a global fitting analysis of the measured transient spectra, three decay time constants, i.e., 1000 ps, are directly obtained in the solvent of acetonitrile. These newly observed lifetime constants are important in clarifying its decay dynamics as well as in providing a criterion for the ultrafast dynamics simulations in 6-azauracil using quantum chemical theories. In combination with previous theoretical works, the main decay channel is proposed: the initially populated S2 decays to S1 through internal conversion in 1000 ps component is due to the decay of the T1 state. A comparison of the excited-state dynamics in different solvents reveals that the decay from S1 to T1 shows a clear dependence on the polarity of the solvents. With higher polarity, the S1 excited state decays faster. This observation is in line with the prediction by Etinski et al. [Phys. Chem. Chem. Phys. 2010, 12, 15665−15671], where a blue-shift of the T1 state potential energy surface leading to an increase of the intersystem crossing rate was proposed. With the new information obtained in the present measurement, a clearer picture of the decay dynamics of 6-azauracil on the S2 excited state is provided.
1. INTRODUCTION DNA and RNA bases are the basic building blocks of the genetic code. In the UV region, the first strong absorption band for all the bases is located near 260 nm. Excitation of this transition is energetic enough to induce bond breaking or conformational change and subsequently leads to mutagenic and carcinogenic effects.1−3 Therefore, the UV-induced photophysics and photochemistry of DNA and RNA bases have attracted a great deal of attention during the past decades.4,5 As a base that only exists in RNA, the ultrafast relaxation mechanism of uracil has received lots of attention.6−10 With the benefit of femtosecond lasers, it is generally believed that the excited state, populated with ∼260 nm excitation, will undergo internal conversion (IC) to the ground state within picoseconds in solution.6−8 According to quantum chemical calculations,9,10 the most probable IC mechanism can be ascribed to the out-of-plane distortion of the CC double bond at the 5-position. It leads to a potential energy surfaces (PESs) crossing between the ground state and the S2 excited state. However, due to the difficulties in extracting the location and geometry of the crossing from the experimental data directly, it is still hard to comprehend this IC process in detail. One possible way to study the characteristics of this IC process is to compare its dynamics with its analogues that have different substituents at different positions. Because of the coupling of the substituents, the out-of-plane distortion of the CC bond © XXXX American Chemical Society
will be modulated, and the dynamics could be changed depending on the substituents and their positions. Therefore, systems such as 5- and 6-aminouracil,11,12 5-fluorouracil,13,14 methyluracil,6,15 and 1-cyclohexyluracil16 are nice candidates for this study, and the role of the CC double bond in the IC process has been gradually revealed. 6-Azauracil (6-AU), where the C atom at the 6-position is replaced by a N atom (Figure 1), is also believed to be a good reference system since no out-of-plane distortion motion in CN bond is expected. Kobayashi et al. have for the first time studied its excited-state dynamics in a time-resolved way using
Figure 1. Molecular structure of 6-AU. Received: September 15, 2015 Revised: December 8, 2015
A
DOI: 10.1021/acs.jpca.5b08975 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
(FemtoPower Compact PRO, Femtolasers Produktions GmbH, 792 nm, 30 fs, 0.8 mJ/pulse, 5 kHz). The output of the amplified pulse is converted to 396 nm with a second harmonic generation BBO and then sum frequency mixed with the fundamental pulse to generate a 264 nm pulse. This pulse is used to excite the molecule to its S2 state. The probe and reference pulses are generated by focusing a small fraction (∼1 μJ) of the 792 nm pulse into a CaF2 plate. White-light continuum is generated, and its spectrum covers from 320 to 800 nm. The pump and probe pulses overlap on the sample solution spatially and temporally, while the sample solution is circulated through a 1 mm thick flow cell. The polarization between the two pulses is set to be magic angle although no noticeable difference is observed while other angles are used. Probe and reference spectrum are measured separately using two linearly CCDs. The wavelengths are calibrated using the different harmonics of the laser, i.e., at 792, 396, and 264 nm, as well as laser diodes with known wavelengths, e.g., 532 nm. The chirp effect of the white-light probe on the time-resolved spectra is corrected on the basis of the optical Kerr effect data. The time resolution of the whole setup is evaluated to be ∼300 fs.
transient absorption spectroscopy and the time-resolved thermal lensing technique.17 They concluded that the singlet → triplet intersystem crossing (ISC) process was predominant for 6-AU with S2 excitation. This observation was totally different from uracil, where IC from S2 to S0 state was found to be the main relaxation pathway. The authors rationalized this significant difference using two candidate mechanisms: (A) N substitution provides another channel to realize the π* → n decay and thus accelerates the ISC process; (B) the C5N6 double bond is more rigid and thus slows down the IC process. Etinski et al. also studied the relaxation dynamics of 6-AU by means of ab initio quantum chemical calculations.18 They found that the ISC process dominates the relaxation pathway, which is in line with the observation of Kobayashi et al. However, they also noticed that the ISC rates from S1 to T1 in uracil and 6-AU were about the same order. With high level quantum chemical calculations, they found that the ultrafast ISC process from S1 to T1 took 125 ps in a vacuum and 34 ps in acetonitrile. Gobbo et al. also calculated the global potential surface of 6-AU.19 Two different pathways that resulted in a population of the T1 state from the initial populated S2 of 6-AU were found. The first one took S1 state as the intermediate state while the second one used the T2 state as the intermediate state. They also proposed that the bifurcation between the two channels weakened the fluorescence from S2 state. Moreover, they found a curve crossing between the S2 and S0 PESs under vacuum. However, there was a barrier of ∼0.6 eV on the reaction pathway, which made this decay channel less likely under 264 nm excitation. Although several studies have been carried out as mentioned above, contradictions still remain and the mechanism of the ultrafast decay of excited 6-AU is far from well understood. In particular, so far, experimental studies of 6-AU only have nanosecond time resolution. Thus, the ultrafast singlet → triplet ISC process is not yet reachable experimentally. On the other hand, a systematic study of the solvent effects on the excited-state dynamics, which will provides more insights into the dynamics, is still lacking. As demonstrated in 5fluorouracil,13,14 the excited-state dynamics in acetonitrile is entirely different from that in water. Moreover, the excited-state lifetime of 6-AU was supposed to be affected by solvent polarity theoretically.18 Therefore, it is important to understand the excited-state dynamics of 6-AU in different solvents with a higher time resolution. In the present paper, the ultrafast decay dynamics of 6-AU on the S2 excited state have been studied with femtosecond transient absorption spectroscopy.20 The ultrafast decay channels, which are not accessible with nanosecond time resolution, have been revealed. In combination with previous calculation results, a comprehensive understanding of the decay dynamics is provided. Furthermore, with the measurements of the solvent effect on the excited-state dynamics, the role of polarity as well as viscosity of the solvents is discussed. These measurements lead to a deeper understanding of the excitedstate dynamics of 6-AU.
3. RESULTS AND DISCUSSION 3.1. Ultrafast Dynamics of 6-AU in Acetonitrile. The steady-state absorption spectra of 6-AU has a broad absorption peak in the UV region.17 According to calculations,17−19 this broad structure consists of a very weak absorption band which centers near 300 nm and a relative strong absorption band that centers around 260 nm. The weak band is due to a transition of a nonbonding electron from HOMO−2 orbital or HOMO−1 orbital to the LUMO orbital, i.e., n → π* transition, and it is assigned to S0 → S1 excitation.17−19 The relative stronger band is assigned to S0 → S2 excitation. It is due to an excitation of a π electron from HOMO orbital to the LUMO orbital, i.e., π → π* transition.17−19 In order to monitor the S2 state dynamics directly, we excite the molecule with a 264 nm pulse. Figures 2a, 2b, and 2c show the transient absorbance change of 6-AU in acetonitrile at different pump−probe delays. Immediately after photoexcitation, an intense transient absorption band which centers at ∼340 nm and a broad peak which lies around 420 nm are observed. From 0.2 to 0.4 ps, the 340 nm band shows a slightly rising while the 420 ± 40 nm region shows a slightly falling. The trends of the spectra change are shown by the arrows in Figure 2. In the subsequent time region from 1 to 10 ps, the absorption band that centers at 340 nm decreases dramatically. At the same time, the 400 ± 40 nm and the 500−600 nm regions exhibit a substantial rising. The trends of the spectra change near 340 nm (as well as ∼420 nm) are opposite in these two time regions (i.e., 0.2−0.4 and 1−10 ps), indicating that two different events happen within 10 ps. The spectra do not show any dramatic change after 10 ps, as shown in Figure 2c. Strong transient-absorption is still observed at 90 ps, showing that molecules are still in the excited state. In order to examine the excited state dynamics quantitatively, we perform a global fitting analysis with multiexponential functions for the measured spectra within the 330−600 nm region. Figure 3 shows the best fit at 340 and 400 nm. For other wavelengths, the results are shown in Figure 1S of the Supporting Information. As shown, at least three time constants, i.e., 1000 ps, are needed to reproduce the observed time profile. The first time constant is measured to be 180 ± 20 fs in our global fitting. However, the
2. EXPERIMENTAL SECTION 2.1. Samples. The samples are purchased from Alfa Aesar and used as received. All the solvents are purchased from Shengshi Huagong (HPLC grade). Fresh samples are prepared in each experiment, and all the measurements are carried out under air-saturated condition. 2.2. Transient Absorption Measurements. The light source of the apparatus is a Ti:sapphire multipass amplifier B
DOI: 10.1021/acs.jpca.5b08975 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
ISC from S2 to T2 and then followed by IC to T1 state. Since both decay pathways result in the same triplet state T1, it is technically difficult to separate them from each other. However, judging from the lifetime constants we have observed in the present study and the coupling efficiency of each step calculated by Etinski et al.18 and Gobbo et al.,19 we plausibly consider the first pathway as the dominant pathway; i.e., the S2 state decays to S1 in
