We measure the spectrum and time-resolved S1 state nonradiative dynamics of the “clamped” cytosine derivative 5,6-trimethylenecytosine (TMCyt) in a supersonic jet, using two-color resonant two-photon ionization (R2PI), UV/UV holeburning, and ns time-resolved pump/delayed ionization. The experiments are complemented with spin-component scaled second-order approximate coupled cluster (SCS-CC2), time-dependent density functional theory, and multi-state second-order perturbation-theory (MS-CASPT2) ab initio calculations. While the R2PI spectrum of cytosine breaks off cm−1 above its band, that of TMCyt extends up to +4400 cm−1 higher, with over a hundred resolved vibronic bands. Thus, clamping the cytosine C5–C6 bond allows us to explore the S1 state vibrations and geometry changes in detail. The TMCyt S1 state out-of-plane vibrations , , and lie below 420 cm−1, and the in-plane , , and vibrational fundamentals appear at 450, 470, and 944 cm−1. vibronic simulations based on SCS-CC2 calculations agree well with experiment if the calculated , , and frequencies are reduced by a factor of 2–3. MS-CASPT2 calculations predict that the ethylene-type conical intersection (CI) increases from +366 cm−1 in cytosine to cm−1 in TMCyt, explaining the long lifetime and extended spectrum. The lowest-energy CI of TMCyt is the “amino out-of-plane” (OPX) intersection, calculated at +4190 cm−1. The experimental internal conversion rate constant at the level is s−1, which is times smaller than in 1-methylcytosine and cytosine. The level relaxes into the T1(3) state by intersystem crossing with s−1. The T1 state energy is measured to lie cm−1 above the S0 state. The lifetime is ns, resulting in an estimated fluorescence quantum yield of %. Intense two-color R2PI spectra of the TMCyt amino-enol tautomers appear above 36 000 cm−1. A sharp S1 ionization threshold is observed for amino-keto TMCyt, yielding an adiabatic ionization energy of eV.
I. INTRODUCTION
A number of femtosecond (fs) pump-probe studies of cytosine (Cyt), 1-methylcytosine (1-MCyt), and 5-fluorocytosine (5-FCyt) in the gas-phase have yielded ultrashort excited-state S1 lifetimes that range from to 3.2 ps.1–5 However, more recent measurements of supersonically jet-cooled amino-keto Cyt, 5-methylcytosine (5-MCyt), and 5-FCyt based on Lorentzian lifetime broadening measurements at their respective electronic origins yielded lower limits on the S1 lifetimes of 45 ps for Cyt and 75 ps for 5-FCyt.6–8 More recent direct lifetime measurements of Cyt, 1-MCyt, and 1-ethylcytosine (1-ECyt) using the picosecond (ps) excitation/ionization delay technique revealed even longer values between ps, which decrease rapidly with increasing vibrational excess energy (Eexc).9
Computational studies of the excited-state dynamics and nonradiative decay of amino-keto Cyt10–20 have located three different conical intersections (CIs) between the ground and lowest excited 1 states. The lowest of these, which dominates the nonradiative decay, is called (Eth)X since the intersection structure is similar to the CI structure of ethylene. This CI is characterized by a puckering of the C6 atom and a twist around the C5–C6 bond, with a H–C5–C6–H torsional angle of 120°.10–13,15,16,19,21,22 The next higher CI involves an N3 out-of-plane bending and a large out-of-plane amino deformation and is called (OP)X.10,16,19,22 The third CI, called , has a semi-planar structure with sp3 hybridization of the C6 atom, shortening of the C2–N3 bond, and stretching of the C2-O bond relative to the ground state minimum.10–16,19,21,22
Zgierski et al. have shown that covalently clamping the C5–C6 bond of Cyt with a trimethylene bridge in 5,6-trimethylenecytosine (TMCyt) increases the S1 state fluorescence lifetime and quantum yield in room-temperature aqueous solution by times relative to Cyt,23–25 to ns and %.26 Their configuration interaction singles (CIS) and second-order approximate coupled cluster (CC2) excited-state calculations predicted that this clamping shifts the (Eth)X conical intersection of cytosine to cm−1 above the S1 minimum, making this CI energetically less accessible.26 The trimethylene bridge in TMCyt hardly affects the -electron framework of Cyt, so the absorption band shifts from nm for Cyt to 280 nm in TMCyt.26 In the clamped cytosine derivative pyrrolocytosine (PC), the C4-amino group (see Fig. 1) and the C5 atom are covalently connected, resulting in a pyrrole ring fused to the Cyt chromophore.27 This extension of the -electron framework significantly shifts the excitation maximum to nm, or about 70–80 nm to the red, compared to Cyt.28,29 For PC, Thompson and co-workers have measured a lifetime of ns and a quantum yield of in pH 7 phosphate buffer.29
The six most stable tautomers of gas-phase 5,6-trimethylenecytosine. The cytosine tautomer labels are analogous to that defined in Ref. 45.
The six most stable tautomers of gas-phase 5,6-trimethylenecytosine. The cytosine tautomer labels are analogous to that defined in Ref. 45.
Intrigued by these observations, we have measured and analyzed the vibronic spectrum of supersonic jet-cooled TMCyt using two-color resonant two-photon ionization (2C-R2PI), UV/UV holeburning, and depletion spectroscopies. We also measured the S1 state lifetime and triplet-state formation kinetics as a function of Eexc, using the nanosecond excitation/ionization delay technique, and report S1 state nonradiative rate constants for internal conversion and intersystem crossing. In addition to the amino-keto tautomer 1 of TMCyt, we have also observed an intense R2PI spectrum that we assign to the transitions of the hydroxy-enol tautomers 2a/2b, see Fig. 1 for the tautomer structures. The measurements are accompanied by calculations of the lowest excited singlet (1) and triplet (3) states of TMCyt using time-dependent (TD) density functional theory (DFT) (TD-CAMB3LYP), spin-component scaled CC2 (SCS-CC2), complete-active-state self-consistent field (CASSCF), and multi-state second-order perturbation-theory (MS-CASPT2) methods.
II. METHODS
A. Experimental methods
TMCyt was synthesized in three steps from adiponitrile according to Ref. 30 and characterized by 1H-NMR spectroscopy, 13C-NMR spectroscopy, and electrospray ionization (ESI) mass spectrometry as described in the supplementary material. The experimental setup for two-color resonant two-photon ionization (2C-R2PI) measurements has been previously described.31,32 Neon carrier gas at 1.8 bar backing pressure was passed through a 20 Hz pulsed nozzle (0.4 mm diameter) heated to 230 °C that contains the TMCyt sample. 2C-R2PI spectra were measured by crossing the skimmed supersonic jet with the unfocused UV excitation and ionization laser beams in the source of a linear time-of-flight mass spectrometer (TOF-MS). Excitation was performed with 0.4–0.6 mJ UV pulses from a frequency-doubled Radiant Dyes NarrowScan D-R dye laser with a bandwidth of 0.05 cm−1 (1500 MHz) or with an Ekspla NT342B optical parametric oscillator (UV-OPO) with a bandwidth of 4–6 cm−1. The frequency scale was calibrated by measuring the fundamental frequency with the WS6 wavemeter.
Ionization light of J/pulse at 225 or 245 nm was produced by an Ekspla NT342B (UV-OPO). The measurements were typically done with 10–30 mV ion signal at the most intense vibronic band. A second frequency-doubled dye laser ( J/pulse) was used as the UV-depletion laser for UV/UV holeburning and depletion experiments. Excitation and ionization were performed as for the R2PI experiments. The depletion laser was fired 300 ns before the excitation/ionization laser pulses.
B. Computational methods
A uniform theoretical treatment of the ground- and excited-state potential energy surfaces of TMCyt is difficult, and we have combined several methods following a similar approach to our recent work on 1-MCyt.33 The electronic ground state of all 14 tautomers and rotamers of TMCyt was first optimized using density functional theory (B3LYP) with the TZVP basis set. The ground state structures of the six most stable tautomers are shown in Fig. 1; these were re-optimized at the correlated level, using the second-order Møller-Plesset (MP2) method in the resolution-of-identity (RI) approximation, the SCS-MP2 method, and the CC2 method in the RI approximation, using the aug-cc-pVTZ basis set.
The adiabatic and vertical transition energies were calculated at the SCS-CC2 level of theory with the aug-cc-pVDZ basis set. Normal-mode calculations were performed for all geometry-optimized structures to assure that they correspond to true potential energy surface minima. The transition energies were also calculated at the MS-CASPT2 level of theory. For the and 1 states, we used TD-CAM-B3LYP/6-311G** optimized geometries, whose MS-CASPT2 energy is lower than that of their CASSCF and SCS-CC2 analogues. For the optimization of (OP)Min, we used the CASSCF(12,12)/6-311G** geometry because the other methods failed to converge to a minimum for that state.
For the calculation of the decay paths and conical intersections, we follow the MS-CASPT2//CASSCF approach, where the paths and structures are optimized at the CASSCF level of theory and the energy profiles along the paths are recalculated at the MS-CASPT2 level to include dynamic correlation energy (see our previous study of 1-MCyt33). For the CASSCF and MS-CASPT2 calculations, we used, respectively, the 6-311G** basis and the ANO-L basis set contracted to 4s3p2d for C, N, and O and 3s2p1d for H. For the CASPT2 calculations, we used an imaginary level shift34 of 0.1 a.u. and the standard ionization potential–electron affinity correction35 of 0.25 a.u.
To calculate the reaction path to (OP)X, we optimized the transition structure (TS) on S1 and obtained the path by combining the intrinsic reaction coordinate36 and initial relaxation direction37 techniques. The calculated barrier includes the vibrational zero-point energy (ZPE) correction, which amounts to −475 cm−1, based on CASSCF frequencies at (1)Min (with 3N − 6 vibrational modes) and at the corresponding TS (including 3N − 7 modes). The CI was optimized using the recently developed double Newton-Raphson algorithm.38 The active space of the CASSCF and MS-CASPT2 calculations was specifically tailored for each path; for details, see the supplementary material. We use (10,10) and (12,12) active spaces for the ethylene- and OP-type paths, respectively. With this approach, the MS-CASPT2 S1/S0 energy gaps at the CI structures were 1973 and 2265 cm−1 (0.24 and 0.28 eV) at (Eth)X and (OP)X, respectively. The path to (Eth)X, which has a sloped topology and does not involve a TS, was approximated with a linear interpolation in internal coordinates.
Density functional theory and CC2 calculations were performed using Turbomole 6.4.39,40 The CASSCF optimizations were performed with a modified version of Gaussian0941 and the MS-CASPT2 calculations with Molcas 7.8.42,43 Vibronic band simulations were done with the PGOPHER program.44 As inputs, we used the SCS-CC2 calculated S0 ground and S1 excited state geometries and the corresponding normal-mode l matrices, employing conformer 1a. Additional diagonal anharmonicity constants44 were included for some modes. The vibronic band intensities are based on full multidimensional Franck-Condon factors, including both mode displacements and mixing between modes (Dushinsky effect).44 The vibronic simulations for conformer 1b are very similar to those for 1a.
III. RESULTS AND DISCUSSION
A. Computational results
1. Tautomers and relative energies
Figure 1 shows the six most stable calculated tautomers and rotamers of TMCyt, and Table I summarizes their relative energies calculated at different levels of theory. All the correlated wave function methods predict the trans-amino-enol 2b tautomer to be the most stable one, with the corresponding cis-rotamer 2a kcal/mol higher. The amino-keto N1H tautomer 1 that is experimentally investigated below exists in conformer 1a, where the amino group and trimethylene ring are out-of-plane in the same direction, denoted Up-up (or Down-down), where the first (capitalized) orientation refers to the NH2 group. In conformer 1b, the NH2 group and trimethylene ring are arranged in opposite directions (Up-down or Down-up). The 1a and 1b forms are close in energy with 1b calculated to lie 3–4 cm−1 above 1a. In the gas phase, both 1a and 1b are less stable than amino-enol conformers by 1.08 kcal/mol (CC2) or 1.54 kcal/mol (SCS-MP2). The B3LYP density functional method predicts the amino-keto N1H tautomer to be the most stable tautomer; however, it is known that this method predicts the order of the cytosine tautomers incorrectly.6,45
Calculated relative energies (in kcal/mol) of 5,6-trimethylenecytosine tautomers and rotamers (see Fig. 1 for definitions). The bold font indicates the most stable (lowest energy) tautomer/rotamer.
. | B3LYP . | MP2 . | CC2 . | SCS-MP2 . |
---|---|---|---|---|
. | TZVP . | aVTZ . | aVTZ . | aug-cc-pVTZ . |
Amino-keto N1H (1a) | 0.00 | 2.69 | 1.08 | 1.54 |
Amino-keto N1H (1b) | 0.02 | 2.69 | 1.09 | 1.55 |
Amino-enol-trans (2b) | 0.42 | 0.00 | 0.00 | 0.00 |
Amino-enol-cis (2a) | 1.01 | 0.56 | 0.57 | 0.57 |
Imino-keto-trans (3a) | 0.58 | 2.87 | 2.05 | 0.60 |
Imino-keto-cis (3b) | 2.20 | 4.55 | 3.68 | 2.20 |
Amino-keto N3H (4) | 5.52 | 8.46 | 6.64 | 7.39 |
Imino-enol-cis-trans N1H | 27.8 | |||
Imino-enol-cis-cis N1H | 18.2 | |||
Imino-enol-trans-trans N1H | 32.5 | |||
Imino-enol-trans-cis N1H | 21.9 | |||
Imino-enol-cis-trans N3H | 15.4 | |||
Imino-enol-cis-cis N3H | 24.6 | |||
Imino-enol-trans-trans N3H | 12.5 | |||
Imino-enol-trans-cis N3H | 20.6 |
. | B3LYP . | MP2 . | CC2 . | SCS-MP2 . |
---|---|---|---|---|
. | TZVP . | aVTZ . | aVTZ . | aug-cc-pVTZ . |
Amino-keto N1H (1a) | 0.00 | 2.69 | 1.08 | 1.54 |
Amino-keto N1H (1b) | 0.02 | 2.69 | 1.09 | 1.55 |
Amino-enol-trans (2b) | 0.42 | 0.00 | 0.00 | 0.00 |
Amino-enol-cis (2a) | 1.01 | 0.56 | 0.57 | 0.57 |
Imino-keto-trans (3a) | 0.58 | 2.87 | 2.05 | 0.60 |
Imino-keto-cis (3b) | 2.20 | 4.55 | 3.68 | 2.20 |
Amino-keto N3H (4) | 5.52 | 8.46 | 6.64 | 7.39 |
Imino-enol-cis-trans N1H | 27.8 | |||
Imino-enol-cis-cis N1H | 18.2 | |||
Imino-enol-trans-trans N1H | 32.5 | |||
Imino-enol-trans-cis N1H | 21.9 | |||
Imino-enol-cis-trans N3H | 15.4 | |||
Imino-enol-cis-cis N3H | 24.6 | |||
Imino-enol-trans-trans N3H | 12.5 | |||
Imino-enol-trans-cis N3H | 20.6 |
The other TMCyt tautomers 2b, 2a, and 4 also exist as pairs of conformers analogous to 1a/1b, but only one form was calculated since the energy difference is expected to be very small. All the imino-enol forms lie kcal/mol above the most stable tautomer 2b at the B3LYP/TZVP level; hence, we do not consider them any further.
2. Electronic transition energies
Table II summarizes the calculated adiabatic and vertical transition energies of the 1a, 1b, 2a, 2b, and 3a conformers predicted by the SCS-CC2 method, together with the MS-CASPT2 transitions for 1a. Both methods are in good agreement, which validates our computational approach. They predict that the S1 state minimum has 1 character. The calculated adiabatic transition of 1a is 31 534 cm−1 (SCS-CC2) and 31 831 cm−1 (MS-CASPT2), in excellent agreement with the experimental value of 31 510 cm−1. The vertical excitation energy of the 1 state with the SCS-CC2 and MS-CASPT2 methods is 36 610 and 36 500 cm−1, respectively. There are two low-lying 1 states which arise from excitations out of the oxygen and nitrogen lone pairs, whose vertical excitation energies are 7000-10 000 cm−1 higher. The structure of the 1 state was optimized at the TD-CAMB3LYP level. It is a minimum on S2 with adiabatic energy of 37 597 cm−1 (MS-CASPT2 single point). The structure optimization of the 1 state did not converge with SCS-CC2 because it reached a region of S2/S1 degeneracy, which is consistent with the small S2/S1 energy gap found at the 1 minimum at the MS-CASPT2 level. Optimization of the 1 state at the CASSCF level leads to (OP)Min, with an adiabatic energy of 33 017 cm−1. The electronic configuration at this structure is analogous to that described in our previous work on 1-MCyt.33
SCS-CC2 and MS-CASPT2 calculated adiabatic and vertical transition energies (in cm−1) and electronic oscillator strengths fel for five tautomers of 5,6-trimethylenecytosine (see Fig. 1).
. | . | SCS-CC2/aug-cc-pVDZ . | . | MS-CASPT2/ANO-L . | . | ||||
---|---|---|---|---|---|---|---|---|---|
Tautomer . | Transition . | Adiab. . | Vert. . | a . | . | Adiab. . | vert.b . | a . | Expt. . |
1a | 1* | 31 534 | 36 610 | 0.091 8 | 31 831 | 36 500 | 0.1174 | 31 510 | |
1 | 42 349 | 0.001 52 | 37 597c | 43 580d | 0.0410 | ||||
46 288e | 0.0032 | ||||||||
(OP) | 33 017f | ||||||||
3* | 27 978 | 30 671 | 27 774g | 31 217 | 24 020–25 140 | ||||
Ion | 65 975 | 65 440 | |||||||
1b | 1* | 31 483 | 36 580 | 0.091 8 | 31 510 | ||||
1* | 42 352 | 0.001 91 | |||||||
3* | 30 654 | 24 020–25 140 | |||||||
Ion | 65 440 | ||||||||
2a | 1* | 38 631h | 43 035 | 0.006 08 | |||||
1* | 34 508 | 38 589 | 0.102 | ||||||
3* | 35 415 | ||||||||
2b | 1* | 39 319h | 43 631 | 0.006 58 | |||||
1* | 35 008 | 38 871 | 0.101 | 35 930 | |||||
3* | 35 446 | ||||||||
3a | 1* | 43 312 | 0.009 25 | ||||||
1* | 41 401 | 41 862 | 0.250 | ||||||
3* | 29 432 |
. | . | SCS-CC2/aug-cc-pVDZ . | . | MS-CASPT2/ANO-L . | . | ||||
---|---|---|---|---|---|---|---|---|---|
Tautomer . | Transition . | Adiab. . | Vert. . | a . | . | Adiab. . | vert.b . | a . | Expt. . |
1a | 1* | 31 534 | 36 610 | 0.091 8 | 31 831 | 36 500 | 0.1174 | 31 510 | |
1 | 42 349 | 0.001 52 | 37 597c | 43 580d | 0.0410 | ||||
46 288e | 0.0032 | ||||||||
(OP) | 33 017f | ||||||||
3* | 27 978 | 30 671 | 27 774g | 31 217 | 24 020–25 140 | ||||
Ion | 65 975 | 65 440 | |||||||
1b | 1* | 31 483 | 36 580 | 0.091 8 | 31 510 | ||||
1* | 42 352 | 0.001 91 | |||||||
3* | 30 654 | 24 020–25 140 | |||||||
Ion | 65 440 | ||||||||
2a | 1* | 38 631h | 43 035 | 0.006 08 | |||||
1* | 34 508 | 38 589 | 0.102 | ||||||
3* | 35 415 | ||||||||
2b | 1* | 39 319h | 43 631 | 0.006 58 | |||||
1* | 35 008 | 38 871 | 0.101 | 35 930 | |||||
3* | 35 446 | ||||||||
3a | 1* | 43 312 | 0.009 25 | ||||||
1* | 41 401 | 41 862 | 0.250 | ||||||
3* | 29 432 |
Vertical excitation from S0 equilibrium geometry, length.
At SCS-CC2 optimized geometry.
minimum optimized at the TD-CAMB3LYP level, S1 energy 35 282 cm−1.
Mixed / with predominant character.
Mixed / with predominant character.
Optimized at the CASSCF level.
Optimized at the TD-CAMB3LYP level.
Not fully converged.
The adiabatic transition energy of conformer 1a is calculated to lie slightly above that of 1b, differing by 51 cm−1 at the SCS-CC2 level. With this method, the transitions of the major tautomers 2b and 2a are calculated to be 1 and to lie at and cm−1, respectively, or about 3500 cm−1 further to blue than the transitions of the 1a/1b conformers. The lowest-energy electronic transition of the imino-keto tautomers 3a and 3b is predicted at 41 400 cm−1 and 40 970 cm−1, respectively. This is above the experimental spectral range covered in this work. On the other hand, the lowest 1 transition of the 4 (N3H) tautomer is predicted to lie very close to that of the 1 (N1H) tautomer. However, tautomer 4 is calculated to be 5.5–5.8 kcal/mol less stable than tautomer 1; hence, we do not expect this tautomer to be observable in the supersonic jet.
3. Ground- and excited-state structures
In the SCS-CC2 S0 optimized structure of 1a, the pyrimidinone framework is Cs symmetric, and the amino group and the trimethylene ring are bent slightly out of the ring plane. In the 1 excited state, the SCS-CC2 and TD-CAMB3LYP methods predict (i) a stronger pyramidalization of the amino group, (ii) an in-plane deformation of the pyrimidinone framework, and (iii) an out-of-plane bend at the C6 atom (see Fig. 1 for the atom numbering). Figure 2 shows the SCS-CC2/aug-cc-pVDZ calculated geometries and geometry changes of TMCyt for both amino-keto N1H conformers. The TD-CAM-B3LYP optimized structure has similar out-of-plane deformations, see Fig. S1 in the supplementary material. This is in line with previous results for 1-MCyt,33 for which both methods predict a substantial deplanarization at the 1 state minimum.
SCS-CC2/aug-cc-pVDZ calculated geometries and geometry changes of amino-keto 5,6-trimethylenecytosine upon 1 excitation (ground state is light-colored and the 1 state is darker). Bond length changes 0.05 Å and bond angle changes 3° are indicated.
SCS-CC2/aug-cc-pVDZ calculated geometries and geometry changes of amino-keto 5,6-trimethylenecytosine upon 1 excitation (ground state is light-colored and the 1 state is darker). Bond length changes 0.05 Å and bond angle changes 3° are indicated.
4. Interconversion between the 1a and 1b isomers
As shown in Table I, three correlated quantum-chemical methods (MP2, CC2, and SCS-MP2) predict the energy difference between conformers 1a and 1b to be very small (1–10 cm−1). At the typical Tvib = 5–7 K in our supersonic jet expansions, the relative population of 1a and 1b should thus be within a factor of 2–3. Given the computed adiabatic transition frequencies of 31 534 cm−1 for 1a and 31 483 cm−1 for 1b (see Table II), we should observe two spectra that are mutually shifted by about 50 cm−1. However, the R2PI and UV/UV holeburning spectra discussed below show only a single ground-state species. The reason for this is the large-amplitude amino-inversion of TMCyt, which interconverts the conformers 1a and 1b.
In the S0 state, the SCS-CC2/aug-cc-pVDZ calculated barrier height between 1a and 1b is cm−1. We calculated the one-dimensional (1D) inversion potential at the same level by incrementing the H–N–C4–N3 and H–N–C4–C5 angles from by in the positive and negative directions, relaxing all other internal coordinates at every point. The resulting 1D potential is shown in Fig. 3. The inversion eigenfunctions in this potential were calculated by numerically solving the 1D vibrational Schrödinger equation. The reduced mass was determined by calculating the SCS-CC2/aug-cc-pVDZ amino-inversion imaginary normal-mode frequency at ; the central five points of the inversion potential were taken to represent the harmonic potential at this angle, and for the 1D calculation was fixed such that the calculated normal-mode and 1D frequencies in this harmonic potential were the same. Figure 3 shows that the lowest-energy = 0 level lies cm−1 above the barrier. Its wave function is delocalized over both the 1a and 1b geometries with its maximum near planarity (). The fact that the vibrational ground state of TMCyt is quasiplanar (delocalized over both 1a and 1b) explains why the UV/UV holeburning spectra, discussed in Sec. III B, reflect the presence of a single ground-state species only. The second amino-inversion level = 1 lies 380 cm−1 higher. It will be collisionally cooled out in the supersonic expansion and will not be considered further.
SCS-CC2/aug-cc-pVDZ calculated S0 state amino inversion potential energy curve of 5,6-trimethylenecytosine plotted along the inversion coordinate , see the text. The minima correspond to the 1a and 1b conformers, which are inequivalent; thus their energies differ by cm−1. The 1a 1b barrier height is 30 cm−1. The wave functions of the v″ = 0 and v″ = 1 inversion levels are shown in blue and red.
SCS-CC2/aug-cc-pVDZ calculated S0 state amino inversion potential energy curve of 5,6-trimethylenecytosine plotted along the inversion coordinate , see the text. The minima correspond to the 1a and 1b conformers, which are inequivalent; thus their energies differ by cm−1. The 1a 1b barrier height is 30 cm−1. The wave functions of the v″ = 0 and v″ = 1 inversion levels are shown in blue and red.
In the S0 state, the planar (Cs symmetric) structure of TMCyt is an index-2 stationary point. Normal-mode analysis at this point yields imaginary frequencies for both the NH2 inversion and trimethylene-ring out-of-plane vibrations. The S0-state barrier to planarity is 307 cm−1 at the SCS-CC2 level. In the 1 excited state, the barrier to planarity is much higher, 1297 cm−1. Four imaginary frequencies are obtained at the Cs stationary point.
B. Experimental results: Two-color resonant two-photon ionization, UV/UV holeburning, and depletion spectra
Figure 4(a) shows a two-color R2PI overview spectrum of jet-cooled TMCyt in the 31 300–38 500 cm−1 range, recorded with the UV-OPO. The transition of the amino-keto 1a/1b tautomer is observed at 31 510 cm−1. At the cm−1 resolution of the UV-OPO, vibronic bands are resolved up to +2100 cm−1 above the band. The R2PI signal extends up 4000 cm−1 above, but due to the high density of vibronic excitations and the moderate cm−1 resolution, the band structure is washed out in this range. In Fig. 4(b), we show a PGOPHER vibronic band simulation based on SCS-CC2 structures and normal modes of TMCyt. They are seen to be in qualitative agreement with the R2PI spectrum and will be discussed in more detail in Sec. III F.
(a) Two-color resonant two-photon ionization spectrum of jet-cooled TMCyt recorded with an UV-OPO as an excitation laser and (b) simulated vibronic spectra (with PGOPHER, plotted in the negative direction) based on the SCS-CC2/aug-cc-pVDZ S0 and S1(1) state calculations.
(a) Two-color resonant two-photon ionization spectrum of jet-cooled TMCyt recorded with an UV-OPO as an excitation laser and (b) simulated vibronic spectra (with PGOPHER, plotted in the negative direction) based on the SCS-CC2/aug-cc-pVDZ S0 and S1(1) state calculations.
Starting at 35 930 cm−1 or 4420 cm−1 above the amino-keto band in Fig. 4(a), further intense narrow-band absorption features are observed. Based on the SCS-CC2 calculations of Sec. III A 2, we assign these to the amino-enol tautomers 2b predicted at 34 508 cm−1 and 2a predicted at 35 008 cm−1, see also Table II.
Figure 5(a) shows a higher-resolution 2C-R2PI spectrum of the amino-keto tautomers over the lowest 1500 cm−1 range (31 400–32 900 cm−1) using a narrow-band frequency-doubled dye laser for excitation. Because of its 0.05 cm−1 bandwidth, the vibronic bands are much better resolved in this spectrum. Detailed vibronic assignments are given in Sec. III C. A high-resolution UV/UV holeburning spectrum is shown in Fig. 5(b) and was recorded with the burn laser at the intense band at cm−1, marked with an asterisk in Fig. 5(a). It reproduces the 2C-R2PI spectrum in Fig. 5(a) in great detail. From this we conclude that all the observed vibronic bands originate from the ground-state level that gives rise to the transition at cm−1. Figure 5(c) shows the corresponding UV/UV depletion spectrum in which the holeburning laser is scanned with the detection laser fixed at the intense cm−1 band. The UV/UV depletion spectrum also reproduces the R2PI spectrum, although the signal/noise ratio is lower than that in the UV holeburning spectrum. At 900 cm−1 above the electronic origin, the widths of the vibronic bands begin to increase, which indicates the onset of rapid non-radiative processes, see Sec. III F. Although no further bands can be observed in the depletion spectrum above +1000 cm−1, the signal remains slightly below the baseline, indicating a constant depletion of the ion signal.
(a) two-color resonant two-photon ionization spectrum of supersonic-jet cooled amino-keto 5,6-trimethylenecytosine (ionization at 245 nm). (b) UV/UV holeburning spectrum with the holeburning laser at the cm−1 band (marked with an asterisk). (c) UV/UV depletion spectrum with the detection laser fixed at the + 59 cm−1 band. The wavenumber scale is relative to the band at cm−1.
(a) two-color resonant two-photon ionization spectrum of supersonic-jet cooled amino-keto 5,6-trimethylenecytosine (ionization at 245 nm). (b) UV/UV holeburning spectrum with the holeburning laser at the cm−1 band (marked with an asterisk). (c) UV/UV depletion spectrum with the detection laser fixed at the + 59 cm−1 band. The wavenumber scale is relative to the band at cm−1.
C. Vibronic band assignments
We first attempted to assign the vibronic bands in the R2PI spectrum of TMCyt in Fig. 5(a) based on the SCS-CC2, CC2, and TD-B3LYP harmonic frequencies of the 1 state given in Table III. The lowest-frequency in-plane vibration is predicted to be cm−1 (SCS-CC2), cm−1 (CC2), or cm−1 (TD-B3LYP); hence, the vibronic bands below cm−1 must arise from out-of-plane vibrations. Experimentally, the two lowest-frequency bands at 38 cm−1 and 59 cm−1 cannot belong to the same progression, so we assign these as fundamentals of the lowest-frequency out-of-plane vibrations and (that is, as and ). Table III shows that the lowest two frequencies calculated with the SCS-CC2, CC2, and TD-B3LYP methods are two to three times larger. Previous experience with SCS-CC2, CC2, and TD-B3LYP excited-state calculations of cytosine derivatives and pyrimidinones has shown that while the in-plane S1 state vibrational frequencies are well reproduced, the calculated out-of-plane vibrational frequencies are often 2-3 times higher than that observed experimentally.6–8,31,33
CC2, TD-B3LYP, and SCS-CC2 calculated normal-modes and wavenumbers (in cm−1) for the lowest 1 excited state of 5,6-trimethylenecytosine.
. | . | . | Conformer 1a . | Conformer 1b . | . | ||||
---|---|---|---|---|---|---|---|---|---|
. | . | . | CC2 . | B3LYP . | SCS-CC2 . | CC2 . | B3LYP . | SCS-CC2 . | . |
. | Irrep.a . | Descriptionb . | aVDZ . | TZVP . | aVDZ . | aVDZ . | TZVP . | aVDZ . | Expt. . |
Boat | 90.9 | 95.7 | 76.6 | 91.7 | 108.3 | 67.3 | 37.6 | ||
Pyrimidine/five-ring twist | 120.1 | 122.7 | 119.8 | 119.8 | 136.6 | 120.9 | |||
Butterfly | 157.8 | 170.5 | 146.6 | 156.2 | 212.8 | 145.9 | 59.2 | ||
N1/five-ring planarization | 212.1 | 203.2 | 191.1 | 212.0 | 221.9 | 186.2 | |||
NH2/C2O | 319.4 | 373.2 | 232.6 | 323.6 | 471.4 | 224.1 | 92.4 | ||
NH2/five-ring | 247.7 | 261.5 | 253.8 | 246.8 | 270.1 | 248.9 | 256.7 | ||
NH2/C6 | 326.5 | 403.6 | 288.0 | 270.2 | 448.6 | 274.9 | 221.0 | ||
NH2/C5/C6 | 367.5 | 343.1 | 336.1 | 327.6 | 372.0 | 345.1 | |||
N1H/C4/ NH2 | 528.6 | 625.5 | 352.8 | 550.6 | 722.3 | 357.7 | |||
N1/ NH2 | 269.2 | 301.8 | 434.7 | 377.6 | 287.6 | 425.2 | |||
6a | 455.3 | 473.2 | 461.4 | 454.8 | 471.4 | 466.4 | 470.5 | ||
6b/N1H/C2O | 469.1 | 497.2 | 471.5 | 470.2 | 536.7 | 453.9 | 449.3 | ||
N1H | 495.5 | 538.4 | 504.9 | 502.6 | 645.2/600.7 | 500.2 | 501.8 | ||
6b?/NH2/N1H/C4/C5 | 559.1 | 579.5 | 553.0 | 532.1 | 579.9 | 552.0 | |||
3/N1H | 603.2 | 640.2 | 617.7 | 603.0 | 640.9 | 618.0 | 615.1 | ||
C7H2/C8H2/C9H2/NH2 | 628.5 | 670.1 | 636.5 | 633.2 | 680.6 | 639.9 | |||
5 | 676.1 | 744.6 | 668.2 | 673.8 | 769.8 | 665.7 | |||
NH2 inversion | 649.1 | 483.7 | 691.3 | 647.6 | 532.7 | 700.5 | |||
N1H/N1C2N3/C2O | 742.2 | 777.0 | 736.6 | 743.2 | 781.1 | 740.3 | |||
N1H/N1C2/N3C4/NH2 | 1056.6 | 1090.9 | 771.1 | 1055.6 | 770.6 | ||||
Five-ring stretch/C7H2/C8H2 | 846.7 | 847.5 | 847.9 | 845.6 | 848.0 | 847.7 | |||
C7H2/C9H2/C8H2 shear/NH2/N3C4 | 879.8 | 883.4 | 868.0 | 882.5 | 905.1 | 866.6 | |||
NH2/N1C2N3/N1H | 858.4 | 926.1 | 901.8 | 858.1 | 888.7 | 900.2 | 944.4 | ||
Five-ring stretch/C2O | 913.1 | 901.2 | 912.9 | 912.5 | 925.0 | 912.4 | |||
C8/C8H/five-ring deformation (as) | 1006.1 | 997.1 | 999.6 | 1006.2 | 1000.8 | 999.2 | |||
Five-ring planar./C7H2/C8H2/C9H2/C6N1C2 | 1033.7 | 1058.6 | 1025.0 | 1032.6 | 1073.0 | 1027.3 | |||
Five-ring planar./C7H2/C8H2/C9H2 | 1030.1 | 1052.1 | 1034.0 | 1031.9 | 1065.4 | 1033.9 | |||
C2O/NH2/C7C8C9 | 919.4 | 1105.9 | 1058.5 | 918.4 | 1092.4 | 1056.6 | |||
C7H2 shear/C8H2 shear/C9H2 shear | 1113.8 | 1135.6 | 1119.2 | 1114.2 | 1171.6 | 1119.3 | |||
N1H/C7H2/C9H2/C8H2 | 1176.8 | 1210.9 | 1184.0 | 1176.5 | 1213.5 | 1183.2 | |||
NH2/N1H/C7H2/C9H2 | 1185.4 | 1196.8 | 1194.1 | 1183.8 | 1193.3 | 1192.0 | |||
C7H2/C8H2/C9H2 | 1216.8 | 1230.0 | 1223.8 | 1218.5 | 1234.6 | 1224.9 | |||
N1C6/C8Ha/C9Ha/N1H | 1231.8 | 1268.4 | 1233.3 | 1232.4 | 1155.7 | 1237.7 | |||
C7H2/C8H2/N1H/C6NH2 | 1325.8 | 1362.7 | 1273.1 | 1325.9 | 1360.4 | 1274.6 | |||
C9H2/C8H2/N1H | 1284.5 | 1313.4 | 1291.6 | 1284.5 | 1291.5 | ||||
C7H2/NH2 | 1273.0 | 1299.8 | 1315.4 | 1272.2 | 1295.9 | 1313.8 | |||
C8Ha | 1311.6 | 1335.7 | 1328.0 | 1311.9 | 1326.7 | ||||
N1C2/C5C6/N1H/C9H2/C7H2 | 1393.8 | 1465.7 | 1392.1 | 1395.2 | 1180.8 | 1401.2 |
. | . | . | Conformer 1a . | Conformer 1b . | . | ||||
---|---|---|---|---|---|---|---|---|---|
. | . | . | CC2 . | B3LYP . | SCS-CC2 . | CC2 . | B3LYP . | SCS-CC2 . | . |
. | Irrep.a . | Descriptionb . | aVDZ . | TZVP . | aVDZ . | aVDZ . | TZVP . | aVDZ . | Expt. . |
Boat | 90.9 | 95.7 | 76.6 | 91.7 | 108.3 | 67.3 | 37.6 | ||
Pyrimidine/five-ring twist | 120.1 | 122.7 | 119.8 | 119.8 | 136.6 | 120.9 | |||
Butterfly | 157.8 | 170.5 | 146.6 | 156.2 | 212.8 | 145.9 | 59.2 | ||
N1/five-ring planarization | 212.1 | 203.2 | 191.1 | 212.0 | 221.9 | 186.2 | |||
NH2/C2O | 319.4 | 373.2 | 232.6 | 323.6 | 471.4 | 224.1 | 92.4 | ||
NH2/five-ring | 247.7 | 261.5 | 253.8 | 246.8 | 270.1 | 248.9 | 256.7 | ||
NH2/C6 | 326.5 | 403.6 | 288.0 | 270.2 | 448.6 | 274.9 | 221.0 | ||
NH2/C5/C6 | 367.5 | 343.1 | 336.1 | 327.6 | 372.0 | 345.1 | |||
N1H/C4/ NH2 | 528.6 | 625.5 | 352.8 | 550.6 | 722.3 | 357.7 | |||
N1/ NH2 | 269.2 | 301.8 | 434.7 | 377.6 | 287.6 | 425.2 | |||
6a | 455.3 | 473.2 | 461.4 | 454.8 | 471.4 | 466.4 | 470.5 | ||
6b/N1H/C2O | 469.1 | 497.2 | 471.5 | 470.2 | 536.7 | 453.9 | 449.3 | ||
N1H | 495.5 | 538.4 | 504.9 | 502.6 | 645.2/600.7 | 500.2 | 501.8 | ||
6b?/NH2/N1H/C4/C5 | 559.1 | 579.5 | 553.0 | 532.1 | 579.9 | 552.0 | |||
3/N1H | 603.2 | 640.2 | 617.7 | 603.0 | 640.9 | 618.0 | 615.1 | ||
C7H2/C8H2/C9H2/NH2 | 628.5 | 670.1 | 636.5 | 633.2 | 680.6 | 639.9 | |||
5 | 676.1 | 744.6 | 668.2 | 673.8 | 769.8 | 665.7 | |||
NH2 inversion | 649.1 | 483.7 | 691.3 | 647.6 | 532.7 | 700.5 | |||
N1H/N1C2N3/C2O | 742.2 | 777.0 | 736.6 | 743.2 | 781.1 | 740.3 | |||
N1H/N1C2/N3C4/NH2 | 1056.6 | 1090.9 | 771.1 | 1055.6 | 770.6 | ||||
Five-ring stretch/C7H2/C8H2 | 846.7 | 847.5 | 847.9 | 845.6 | 848.0 | 847.7 | |||
C7H2/C9H2/C8H2 shear/NH2/N3C4 | 879.8 | 883.4 | 868.0 | 882.5 | 905.1 | 866.6 | |||
NH2/N1C2N3/N1H | 858.4 | 926.1 | 901.8 | 858.1 | 888.7 | 900.2 | 944.4 | ||
Five-ring stretch/C2O | 913.1 | 901.2 | 912.9 | 912.5 | 925.0 | 912.4 | |||
C8/C8H/five-ring deformation (as) | 1006.1 | 997.1 | 999.6 | 1006.2 | 1000.8 | 999.2 | |||
Five-ring planar./C7H2/C8H2/C9H2/C6N1C2 | 1033.7 | 1058.6 | 1025.0 | 1032.6 | 1073.0 | 1027.3 | |||
Five-ring planar./C7H2/C8H2/C9H2 | 1030.1 | 1052.1 | 1034.0 | 1031.9 | 1065.4 | 1033.9 | |||
C2O/NH2/C7C8C9 | 919.4 | 1105.9 | 1058.5 | 918.4 | 1092.4 | 1056.6 | |||
C7H2 shear/C8H2 shear/C9H2 shear | 1113.8 | 1135.6 | 1119.2 | 1114.2 | 1171.6 | 1119.3 | |||
N1H/C7H2/C9H2/C8H2 | 1176.8 | 1210.9 | 1184.0 | 1176.5 | 1213.5 | 1183.2 | |||
NH2/N1H/C7H2/C9H2 | 1185.4 | 1196.8 | 1194.1 | 1183.8 | 1193.3 | 1192.0 | |||
C7H2/C8H2/C9H2 | 1216.8 | 1230.0 | 1223.8 | 1218.5 | 1234.6 | 1224.9 | |||
N1C6/C8Ha/C9Ha/N1H | 1231.8 | 1268.4 | 1233.3 | 1232.4 | 1155.7 | 1237.7 | |||
C7H2/C8H2/N1H/C6NH2 | 1325.8 | 1362.7 | 1273.1 | 1325.9 | 1360.4 | 1274.6 | |||
C9H2/C8H2/N1H | 1284.5 | 1313.4 | 1291.6 | 1284.5 | 1291.5 | ||||
C7H2/NH2 | 1273.0 | 1299.8 | 1315.4 | 1272.2 | 1295.9 | 1313.8 | |||
C8Ha | 1311.6 | 1335.7 | 1328.0 | 1311.9 | 1326.7 | ||||
N1C2/C5C6/N1H/C9H2/C7H2 | 1393.8 | 1465.7 | 1392.1 | 1395.2 | 1180.8 | 1401.2 |
Irreducible representation in the Cs point group.
Based on SCS-CC2/aug-cc-pVDZ eigenvectors; = stretching vibration; = in-plane bending vibration; = out-of-plane bending vibration; = scissoring vibration; = in-plane rocking vibration; = out-of-plane wagging vibration; = torsion vibration.
For the PGOPHER44 vibronic band simulations (see Sec. II B), we therefore decreased the out-of-plane frequencies to the experimental values. Figures 6(a)–6(c) show the simulated vibronic bands in red for the 0–420, 420–870, and 870–1320 cm−1 sections of the spectrum and compare these to the high-resolution 2C-R2PI spectrum in black. We first fitted the S1 state in-plane vibrational frequencies. The SCS-CC2 calculation predicts the lowest four in-plane fundamentals , , , and at 254, 461, 472, and 618 cm−1, respectively, see Table III. These normal-mode eigenvectors are shown in Fig. 7. The and normal-modes correspond to the and in-plane vibrations that are characteristic of the spectra of benzene and its derivatives. We therefore assigned the bands at 449 cm−1 and 471 cm−1 as and , respectively, see Fig. 6(b); the order of these two vibrations was interchanged to obtain a better fit with the experimental R2PI spectrum. The transition was fitted to the band at 257 cm−1; its intensity is rather small and it does not contribute further to the spectrum. The band at 615 cm−1 was assigned to the fundamental. The band at 944 cm−1 is assigned as the in-plane fundamental , as the overtone had no intensity, see Fig. 6(c).
2C-R2PI spectrum of jet-cooled TMCyt (upper traces, in black) and PGOPHER simulated vibronic spectrum (lower red traces, plotted in the negative direction) for (a) the region −30–420 cm−1, (b) the region 420–870 cm−1 (enhanced by factor 2), and (c) the region 870–1320 cm−1 (enhanced by factor 1.5). For details of the PGOPHER simulation see Fig. 4.
2C-R2PI spectrum of jet-cooled TMCyt (upper traces, in black) and PGOPHER simulated vibronic spectrum (lower red traces, plotted in the negative direction) for (a) the region −30–420 cm−1, (b) the region 420–870 cm−1 (enhanced by factor 2), and (c) the region 870–1320 cm−1 (enhanced by factor 1.5). For details of the PGOPHER simulation see Fig. 4.
SCS-CC2/aug-cc-pVDZ normal-mode eigenvectors of the four lowest-frequency in-plane vibrations of 5,6-trimethylenecytosine.
SCS-CC2/aug-cc-pVDZ normal-mode eigenvectors of the four lowest-frequency in-plane vibrations of 5,6-trimethylenecytosine.
We then fitted the out-of-plane vibrations, see Fig. 6(a). The weak band at 38 cm−1 is assigned as the fundamental. Since the and vibrations involve structural changes of the trimethylene ring, see Table III, and hardly appear in the simulation, the intense 59 cm−1 band is assigned as the “butterfly” vibrational fundamental. The overtone was fitted to the band at 126 cm−1. The fundamentals of and were fitted to the bands at 93 cm−1 and 221 cm−1, respectively. The out-of-plane normal-mode eigenvectors , , , and are shown in Fig. 8. Note that the SCS-CC2, CC2, and TD-B3LYP harmonic frequencies in Table III differ from the fitted frequencies (Table IV) by a factor of 2–3, indicating that the S1 state potential-energy surface is much flatter and more anharmonic along these coordinates than that predicted by the excited-state calculations.
SCS-CC2/aug-cc-pVDZ normal-mode eigenvectors of the state out-of-plane modes of 5,6-trimethylenecytosine.
SCS-CC2/aug-cc-pVDZ normal-mode eigenvectors of the state out-of-plane modes of 5,6-trimethylenecytosine.
Experimental 1 state vibrational wavenumbers of 5,6-trimethylenecytosine (in cm−1) measured by 2C-R2PI spectroscopy.
Assignment . | Frequency . | . | Frequency . | . | Frequency . |
---|---|---|---|---|---|
Region I | Region II | Region III | |||
(31 510.2) | 449.3 | + | 919.8 | ||
37.7 | 470.6 | 944.4 | |||
59.3 | + | 488.2 | + + | 976.5 | |
92.5 | + | 1002.5 | |||
+ | 97.7 | + | 508.0 | ||
125.7 | + | 514.1 | + + | 1049.7 | |
+ | 128.7 | + | 529.6 | + | 1067.1 |
+ | 142.4 | + | 1085.1 | ||
+ | 165.4 | + | 575.6 | + | 1164.5 |
+ + | 190.3 | + | 596.0 | + + | 1225.6 |
? | 212.7 | 615.1 | + | 1392.5 | |
+ | 218.4 | + | 1415.8 | ||
221.0 | + | 678.7 | + + | 1436.0 | |
+ | 274.1 | + | 693.9 | 1447.0 | |
+ | 739.4 | 1473.8 |
Assignment . | Frequency . | . | Frequency . | . | Frequency . |
---|---|---|---|---|---|
Region I | Region II | Region III | |||
(31 510.2) | 449.3 | + | 919.8 | ||
37.7 | 470.6 | 944.4 | |||
59.3 | + | 488.2 | + + | 976.5 | |
92.5 | + | 1002.5 | |||
+ | 97.7 | + | 508.0 | ||
125.7 | + | 514.1 | + + | 1049.7 | |
+ | 128.7 | + | 529.6 | + | 1067.1 |
+ | 142.4 | + | 1085.1 | ||
+ | 165.4 | + | 575.6 | + | 1164.5 |
+ + | 190.3 | + | 596.0 | + + | 1225.6 |
? | 212.7 | 615.1 | + | 1392.5 | |
+ | 218.4 | + | 1415.8 | ||
221.0 | + | 678.7 | + + | 1436.0 | |
+ | 274.1 | + | 693.9 | 1447.0 | |
+ | 739.4 | 1473.8 |
D. Photoionization efficiency curves
Figure 9 shows the photoionization efficiency (PIE) curves of the S1(1) state, which were recorded at 0 ns delay of the ionization laser, and of a long-lived state, which was recorded at 50 ns delay. The PIE curve of the long-lived state shown in Fig. 9 is scaled according to the relative signal heights discussed in Sec. III E, where the T1 ion signal reaches 25% of the S1 signal when ionizing at 225 nm.
Photoionization efficiency curves of 5,6-trimethylenecytosine following excitation at the S1 band (a) with prompt ionization (0 ns delay); the step-like adiabatic photoionization threshold is shown in the inset (5x). (b) PIE curve with the ionization laser delayed by 50 ns, relative to the excitation. The uncertainty of the T1 photoionization threshold is indicated with a blue bar.
Photoionization efficiency curves of 5,6-trimethylenecytosine following excitation at the S1 band (a) with prompt ionization (0 ns delay); the step-like adiabatic photoionization threshold is shown in the inset (5x). (b) PIE curve with the ionization laser delayed by 50 ns, relative to the excitation. The uncertainty of the T1 photoionization threshold is indicated with a blue bar.
The PIE curve of the state in Fig. 9 exhibits a step-like ionization threshold at cm−1, indicating that the geometry change between the level of the S1(1) state and the TMCyt+ ion ground state D0 is small. The Franck-Condon factor for adiabatic ionization from the S1 state is sufficiently large, so the adiabatic ionization energy (AIE) threshold can be observed. The sum of the excitation energy of 31 510 cm−1 and the PIE threshold in Fig. 9 is cm−1, giving an AIE eV. The SCS-CC2 calculated AIE = 8.18 eV of tautomer 1a is in good agreement with this value (see Table II).
The delayed-ionization PIE curve of the long-lived state shown in Fig. 9(b) is relatively noisy; since the UV spectrum of the TMCyt amino-enol forms begins around cm−1, this contribution to the signal had to be subtracted. The PIE curve exhibits a gradual signal onset at 40 320 cm−1 followed by a slow rise. We interpret the long-lived state as the lowest triplet state T1 and this slow onset as photoionization of the hot vibrational levels of T1 that are formed by intersystem crossing (ISC); the energy difference is converted to vibrational energy of the T1 state during the ISC process. The signal onset at 40 320 cm−1 is thus interpreted as the lower limit to the AIE of the T1 state. The upper limit to the AIE is estimated by back-extrapolation of the linear part of the PIE curve to the zero-signal line at 41 400 cm−1. Subtracting these two values from the AIE of the S1(1) state ( cm−1) places the T1 state between 24 020 cm−1 and 25 140 cm−1 above the S0 ground state. The SCS-CC2 calculations (see Table II) support this assignment; the calculated adiabatic T1 state adiabatic energy is cm−1, whereas the alternative of a dark 1 state can be discarded because its estimated energy is much higher, 37 597 cm−1, see Table II.
E. Ns pump/ionization delay scan measurements and nonradiative kinetics
We measured the excited-state lifetime and nonradiative kinetics of TMCyt using ns laser pump/delayed ionization measurements by ionizing at 225 nm. The convolution of the pulse widths of the pump and ionization laser yields a Gaussian instrument response function (IRF) with a full width at half-maximum (FWHM) of 4.2 ns. We modeled the S1 (1) state kinetics as
where krad is the radiative decay rate. The SCS-CC2 calculated oscillator strength of TMCyt is fel = 0.0918, giving ns or s−1. This value is in good agreement with the ns that Zgierski et al. estimated from the integrated absorption spectrum of TMCyt in aqueous solution.26 The S1 state is assumed to decay nonradiatively to S0 by internal conversion (IC) with the rate constant and by intersystem crossing (ISC) to the T1 state with the rate constant . T1 is assumed to relax to S0 by reverse ISC and also by phosphorescence; these two pathways are combined into a single rate constant kT,
However, kT is very low ( s) and we cannot determine it by delay measurements on the ns time scale, so it is set to zero. The simulated time-dependent concentrations [S1] and [T1] were convoluted with the IRF and were least-squares fitted to the experimental pump/ionization signal traces.
Note that because of the 4.2 ns width of the IRF, which is similar to the inverse of the kIC and kISC rate constants, the ratio of the ionization efficiencies of molecules in the S1 and T1 states, (S1):(T1), can only be estimated within certain limits discussed below. If the width of the two laser pulses was significantly shorter than the inverse of the kIC and kISC rates, then the experimental pump/ionization transient would exhibit a much more intense S1 signal that would peak close to 100% on the scale as shown in Fig. 10, and the observed S1:T1 signal ratio would be correspondingly larger.
Ns laser excitation/delayed ionization lifetime measurements of TMCyt (in black). Excitation at the band, ionization at 225 nm. The kinetic fits assume that the ratios of S1 and T1 ionization cross sections are (a) 1:1, (b) 4:1, and (c) 6.4:1, for details see the text. The S1 state signal contribution is plotted in blue and that of the T1 state is in yellow; the fit of the total ion signal is in red.
Ns laser excitation/delayed ionization lifetime measurements of TMCyt (in black). Excitation at the band, ionization at 225 nm. The kinetic fits assume that the ratios of S1 and T1 ionization cross sections are (a) 1:1, (b) 4:1, and (c) 6.4:1, for details see the text. The S1 state signal contribution is plotted in blue and that of the T1 state is in yellow; the fit of the total ion signal is in red.
In Figs. 10(a)–10(c), we show the experimental pump/ionization transient with excitation at the band and ionization at 225 nm, marked by a dashed vertical line in Fig. 9. This transient is fitted for three different assumptions for the ionization efficiency ratio (S1):(T1). In Fig. 10(a), we assume (S1):(T1) = 1, giving the nonradiative rate constants s−1 and s−1. Note, however, that this ratio is unrealistically low since ionization at 225 nm is 10 000 cm−1 above the S1 ionization threshold but only 2700 cm−1 above the T1 ionization threshold. For the fit in Fig. 10(b), we assume that the ionization efficiency ratio (S1):(T1) = 4, which is the apparent experimental ratio between the S1 and T1 ion signals at 225 nm shown in Fig. 9 and between the ion signals at 0 ns delay and 40 ns delay shown in Fig. 10. This fit gives the nonradiative rate constants s−1 and s−1. If—as the other limiting case—we assume kIC to be zero and fit kISC and the (S1):(T1) ratio, we obtain the fit curves shown in Fig. 10(c). The resulting (S1):(T1) = 6.4 is the maximum possible ratio, and the fitted s−1 is an upper limit for the ISC rate.
These IC and ISC rate constants of TMCyt can be compared to those of 1-MCyt, which are s−1 and s−1 near the level.33 The main difference lies in the decrease of kIC by a factor of 10–20. The ISC rate constant probably changes little upon rigidization of the pyrimidinone, but the uncertainty is large. Thus the increase in excited-state lifetime at the band upon clamping the C5–C6 bond originates mainly from the decrease of the IC rate.
The pump/ionization transients were also measured at an ionization wavelength of 245 nm, which is the same wavelength as used to record the 2C-R2PI spectra. The measured band transient was well fitted with the three sets of kIC and kISC constants that correspond to Figs. 10(a)–10(c). However, Fig. 9 shows that ionization of the T1 state at 245 nm is very inefficient; thus, the (S1):(T1) ratio was re-fitted and is 15.5 times larger that for ionization at 225 nm. These fits are shown in Figs. S3(a)–S3(c) of the supplementary material. Ns pump/ionization transients were also measured for the bands at , , and cm−1, but only with ionization at 245 nm, see Figs. S3(d)–S3(f) of the supplementary material. These transients were fitted with a fixed (S1):(T1) = 15.5, which corresponds to assuming (S1):(T1) = 1 and T1 states at 225 nm. All fitted kIC and kISC values assuming (S1):(T1) = 1 at 225 nm are collected in Table V.
Internal conversion (IC) and intersystem crossing (ISC) rate constants, decay lifetimes, fluorescence quantum yields , and ISC quantum yields , from fits to the ns excitation/ionization transients in Fig. 10 and Fig. S3 of the supplementary material, assuming the relative ionization efficiencies of the S1 and T1 states at 225 nm to be equal.
Band (cm−1) . | kIC/108 (s−1) . | kISC/107 (s−1) . | Lifetime > (ns) . | . | . |
---|---|---|---|---|---|
2.2 | 4.1 | 2.9 0.2 | 0.24 | 0.12 | |
+530 | 3.4 | 6.6 | 2.0 0.3 | 0.17 | 0.13 |
+1174 | 4.6 | 9.4 | 1.6 0.2 | 0.13 | 0.15 |
+1646 | 12 | 12 | 0.7 0.4 | 0.06 | 0.09 |
Band (cm−1) . | kIC/108 (s−1) . | kISC/107 (s−1) . | Lifetime > (ns) . | . | . |
---|---|---|---|---|---|
2.2 | 4.1 | 2.9 0.2 | 0.24 | 0.12 | |
+530 | 3.4 | 6.6 | 2.0 0.3 | 0.17 | 0.13 |
+1174 | 4.6 | 9.4 | 1.6 0.2 | 0.13 | 0.15 |
+1646 | 12 | 12 | 0.7 0.4 | 0.06 | 0.09 |
Summarizing, one sees that although the ns time resolution of the pump/ionization transient measurement and the unknown ratio (S1):(T1) lead to considerable uncertainty, kIC is determined within a factor of 2.5 between and s−1. Similarly, the limits of the ISC rate constant are determined within a factor of four as to s−1. For all three fits, the lifetime at the band is ns. Given the calculated radiative rate constant s−1 and that , one finds that the fluorescence quantum yield of TMCyt is %. This value does not depend on the exact kIC and kISC rate constants. For TMCyt in room-temperature aqueous solution, Zgierski et al. determined % from the lifetime of ns.26 The fact that the fluorescence quantum yield at room temperature is lower than that at the low temperature in the supersonic jet is very reasonable and to be expected from the increase of kIC with increasing vibrational energy, as is documented in Table V.
F. The conical intersections
In contrast to the vibronic spectra of Cyt and its derivatives 1-MCyt, 5-MCyt, and 5-FCyt,6–8,33 which exhibit sharp break-offs at 450–1200 cm−1 above the bands, indicating the onset of an ultrafast process, the 2C-R2PI spectrum of TMCyt 1a/1b extends up to 4400 cm−1 above the band and does not show a spectral break-off. The vibronic bands either merge or become diffuse at cm−1 above the band of the amino-keto tautomer. To investigate the reason for the broadening, we modeled the complete vibronic spectrum for TMCyt using PGOPHER 8.0;44 the simulated spectrum is shown in Fig. 4(b). In addition to the nine optically active vibrational modes , , , , , , , , and that were employed for the simulation in Fig. 6 in Sec. III C, we included the fundamental excitations of all vibrations with calculated Franck-Condon factors % of the band. These are the in-plane vibrations , , , , and , and the out-of-plane vibration. These frequencies were not fitted to experimental transitions but were taken from the SCS-CC2 calculations. The overtones and combination tones of these six vibrations could not be included because of the limited array sizes of PGOPHER.
A Gaussian line shape with a FWHM of 5 cm−1 was employed, reflecting the bandwidth of the UV-OPO. When setting the Lorentzian linewidth contribution to zero, the simulated spectrum exhibits resolved vibronic bands up to +4400 cm−1. If we include a Lorentzian linewidth contribution cm−1 in the simulation, which corresponds to a lifetime of 1 ps, we see in Fig. 4(b) that the bands broaden and merge into a semi-continuous background that is similar to the experimental spectrum in Fig. 4(a). This suggests that the broadening of the spectrum at excess energies above +2100 cm−1 does not reflect just spectral congestion but arises from a decrease in the excited-state lifetime.
To account for the additional broadening observed in the experimental spectrum, we have calculated the two most energetically favorable excited-state decay paths that are analogous to those for cytosine and 1-MCyt.10,16,18,19,46–49 According to expectations and in line with the calculations of Zgierski et al.,26 the access to the ethylene-type intersection is hindered by the trimethylene modification. The calculated energy of the (Eth)X CI is approximately 6800 cm−1 relative to the transition. The path from 1 to that CI has a sloped topology, and the barrier for the decay is given by the energy of the CI itself, see Fig. S2 in the supplementary material.
The energetically favored decay path involves out-of-plane deformation of N3 and the amino group. The calculated energy profile along this path is shown in Fig. 11. The path leads from the structure through a transition state (TS) to a second minimum, (OP)Min, which is similar to that previously characterized for Cyt and 1-MCyt.10,16,18,19,46–49 The MS-CASPT2 barrier over the TS is 1935 cm−1, and the energy of (OP)Min relative to 1 is 1056 cm−1. The decay path leads further to the (OP)X CI, which has a relative energy of approximately 4300 cm−1. The (OP)Min and (OP)X structures are characterized by a large out-of-plane bending of N3, with a C2–N3–C4–C5 angle of and , respectively. The ring puckering and out-of-plane bending of the amino group also increase from (OP)Min to (OP)X (see the O8–C2–N1–C6 and N7–C4–C5–C6 angles in Fig. 12), which is consistent with the decay path where (OP)Min lies before the (OP)X CI.
MS-CASPT2 energy profile of the S0 and S1 singlet states along the CASSCF optimized paths for radiationless decay on S1 from (1)Min to (OP)X through (OP)Min. Blue squares: S1; blue dots: S0.
MS-CASPT2 energy profile of the S0 and S1 singlet states along the CASSCF optimized paths for radiationless decay on S1 from (1)Min to (OP)X through (OP)Min. Blue squares: S1; blue dots: S0.
(a) Structure of the (OP)Min minimum in the S1 energy surface of 5,6-trimethylenecytosine. (b) Structure of the (OP)X conical intersection.
(a) Structure of the (OP)Min minimum in the S1 energy surface of 5,6-trimethylenecytosine. (b) Structure of the (OP)X conical intersection.
The fact that broadening of the vibronic bands in the R2PI spectrum sets in at around cm−1, but a semi-continuous spectrum continues up to 4400 cm−1 above the electronic origin, is in qualitative agreement with the calculated decay path topology. We interpret the additional broadening beyond cm−1 due to the coupling between the vibrations belonging to the - and the OPMin-minima below the barrier. The density of vibronic states belonging to both minima rises enormously when the energy exceeds this barrier (MS-CASPT2 barrier 1935 cm−1). The semi-continuous spectrum that reaches up to at least 4400 cm−1 is also in good agreement with the calculated CI at 4300 cm−1.
IV. CONCLUSIONS
We have measured the UV vibronic spectra of jet-cooled amino-keto 5,6-trimethylenecytosine (TMCyt) using two-color resonant two-photon ionization, UV/UV holeburning, and depletion spectroscopies. The band is identified at 31 510 cm−1. The lowest 400 cm−1 of the spectrum is dominated by fundamentals, overtone excitations, and combination bands of four out-of-plane vibrations. Based on the energetic sequence of the SCS-CC2 calculated vibrational frequencies and on their predicted Franck-Condon factors, we assign these as , , , and . Similar to the spectra of cytosine, 5-methylcytosine, and 5-fluorocytosine,6–8 the longest vibronic progression is observed for the butterfly vibration . Combination progressions in are also built on the in-plane vibrational fundamentals , , , and . In contrast to unsubstituted cytosine, whose spectrum breaks off above cm−1, the R2PI spectrum of TMCyt extends to cm−1 above, with more than 100 resolved vibronic bands. This is the most extended spectrum of any cytosine derivative measured so far. We have also observed the R2PI spectra of the amino-enol tautomers 2a and 2b starting at cm−1, but these will be discussed elsewhere.
Sharp vibronic bands can be observed up to +2100 cm−1 above the band. Hence, bridging of the C5–C6 bond with the trimethylene ring strongly raises the barrier to the ethylene-type (Eth)X conical intersection. Above +2100 cm−1, a semi-continuous R2PI spectrum is observed up to at least +4400 cm−1. The vibronic band simulation performed with the PGOPHER program44 nicely reproduces the vibronic band structure and intensities of the R2PI spectrum up to +1320 cm−1. Towards higher frequencies, the simulations predict resolved vibronic transitions, whereas in the R2PI spectrum, an increased density of bands leads to an intense continuous signal.
From a mechanistic perspective, our computational work shows that, by blocking the twist of the C5–C6 bond, we not only change the energetically favored decay path but also the topology. In Cyt and 1-MCyt, the decay path leads from the 1 minimum via a TS to the ethylene-type (Eth)X CI. The CI can be reached as soon as enough energy is available to go over the TS, and this is observed as a sharp break-off of the R2PI spectrum above cm−1 in these systems. In contrast, in TMCyt, the lowest CI is the amino out-of-plane bend (OP)X and the (Eth)X CI is raised cm−1 above the S1 vibrationless level. The path to (OP)X involves an additional minimum that lies before the intersection. As a consequence, the R2PI spectrum does not completely break off when enough energy is available to go over the TS. This suggests that the broad, shapeless spectral region between 2100 and 4300 cm−1 is a signature of the calculated topology.
The excited-state lifetime of amino-keto TMCyt at the band is ns, which is a fourfold increase relative to that of cytosine at its band. Additionally, the lifetime drops off much more slowly with increasing vibrational excess energy, being ns even at a vibrational excess energy Eexc = 1174 cm−1. From the calculated S1 state radiative lifetime and experimental lifetime , we infer that the fluorescence quantum yield at the v′ = 0 level is %, which makes TMCyt the strongest fluorescing cytosine derivative in the gas phase known to date. drops to % at Eexc = 1646 cm−1. These fluorescence lifetimes and quantum yields are in qualitative agreement with the ns and % values that Zgierski et al. determined for TMCyt in room-temperature aqueous solution.26 The availability of a strongly fluorescent gas-phase cytosine derivative opens exciting new research opportunities based on fluorescence measurements.
SUPPLEMENTARY MATERIAL
See supplementary material for the synthesis of 5,6-trimethylenecytosine, additional computational details on the MS-CASPT2 calculations, and additional ns pump/ionization transients.
ACKNOWLEDGMENTS
The Bern group acknowledges support by the Schweizerische Nationalfonds (Project No. 200020-152816). L.B. acknowledges financial support from the Spanish Ministerio de Economía y Competitividad (CTQ2015-69363-P) and the Generalitat de Catalunya (2014SGR-2012).
REFERENCES
The thresholds for SCF and one-electron density convergence were set to 10−9 a.u. and 10−8 a.u., respectively. The convergence thresholds for all structure optimizations were set to 10−8 a.u. for the energy change, a.u. for the maximum displacement element, 10−6 a.u. for the maximum gradient element, a.u. for the RMS displacement, and 10−6 a.u. for the RMS gradient.