Two-color Resonant Two-photon Ionization Research Articles

Positive cooperativity in binding: Spectroscopic characterization and dissociation energy of the anisole…(CH4)2 complex

We have previously reported on the spectroscopy and binding energy of the anisole…methane complex, which exhibits a dual mode of binding that involves both CH/O and CH/π interactions. In this work, we seek to examine cooperativity in binding through a study of the isolated anisole-(methane)2 complex, using a combination of experiments that include mass-selected two-color resonant two-photon ionization spectroscopy (2CR2PI), two-color appearance potential (2CAP) measurements, and velocity mapped ion imaging (VMI) augmented with a complementary theoretical characterization. Using 2CAP and VMI, we derive the dissociation energies of the complex in ground (S0), excited (S1), and cation radical (D0) states. The experimental values from the two methods are in excellent agreement and are compared with selected theoretical values calculated using DFT and ab initio methods. The data show that the dissociation energy increases by some 10 % for the second methane relative to the first, with this trend being consistent across all three electronic states, indicating a cooperative binding effect where the initial solvation turns on binding of a second methane onto the opposite face. This lies in contrast to the aniline-(methane)2 complex, where recent studies have shown a negativity cooperativity, and these trends are examined.

Laser spectroscopic characterization of supersonic jet cooled 2,7-diazaindole.

We report the first gas phase comprehensive study of the electronic spectroscopy of 2,7-diazaindole molecule in the ground and excited states. Single vibronic level fluorescence spectroscopy (SVLF) was performed to determine the ground state vibrations of the molecule, which depicted a large Franck-Condon activity beyond 2600 cm-1. For the excited state characterization, laser-induced fluorescence (LIF) and two-color resonant two-photon ionization spectroscopy (2C-R2PI) were performed. The band origin (000) for S1 ← S0 transition appeared at 33910 ± 1 cm-1 which was red shifted by 718 cm-1 and 1322 cm-1 compared to that of 7-azaindole and indole respectively. The Franck-Condon active vibrational modes in the spectra were seen till the (000) + 1600 cm-1 region. The IR-UV hole burning spectroscopy confirmed the absence of any other isomeric species in the molecular beam. The ionization energy (IE) of the molecule was measured as 8.921 ± 0.001 eV, recorded using photoionization efficiency spectroscopy. The above IE value was significantly higher than that of the related indole derivatives, suggesting the higher photostability of the 27DAI molecule due to N(2) insertion. The ground and excited state N-H stretching frequencies of the molecule were determined using fluorescence-dip infrared spectroscopy (FDIR) and resonant ion-dip infrared spectroscopy (IDIR), and the values are 3523 and 3467 cm-1, respectively. The lower value of νNH in the electronic excited state implied the increased photoacidity of the group. A comparative analysis of the experimental LIF/2C-R2PI spectra was done against Franck-Condon simulated spectra at three different levels of theory. The vibrational frequencies calculated at B3LYP-D4/def2-TZVPP showed the most accurate prediction in comparison with the experimentally detected symmetric modes in the ground state. However, in the excited state, the lower energy asymmetric modes simulated at the B3LYP/def-SVP level of theory provided the best agreement with the experiment. This is most probably due to the distortion observed at the pyrazolyl ring leading to the appearance of asymmetric vibrational modes. The above study highlights the possibility to appropriately tune the excitation wavelengths as well as alter the photostability of the organic chromophores via additional N-insertion in the molecular systems.

Optical Identification of the Resonance-Stabilized para-Ethynylbenzyl Radical

We report the spectroscopic observation of the jet-cooled para-ethynylbenzyl (PEB) radical, a resonance-stabilized isomer of C9H7. The radical was produced in a discharge of p-ethynyltoluene diluted in argon and probed by resonant two-color two-photon ionization (R2C2PI) spectroscopy. The origin of the D0(2B1)-D1(2B1) transition of PEB appears at 19,506 cm-1. A resonant two-color ion-yield scan reveals an adiabatic ionization energy (AIE) of 7.177(1) eV, which is almost symmetrically bracketed by CBS-QB3 and B3LYP/6-311G++(d,p) calculations. The electronic spectrum exhibits pervasive Fermi resonances, in that most a1 fundamentals are accompanied by similarly intense overtones or combination bands of non-totally symmetric modes that would carry little intensity in the harmonic approximation. Under the same experimental conditions, the m/z = 115 R2C2PI spectrum of the p-ethynyltoluene discharge also exhibits contributions from the m-ethynylbenzyl and 1-phenylpropargyl radicals. The former, like PEB, is observed herein for the first time, and its identity is confirmed by measurement and calculation of its AIE and D0-D1 origin transition energy; the latter is identified by comparison with its known electronic spectrum (J. Am. Chem. Soc., 2008, 130, 3137-3142). Both species are found to co-exist with PEB at levels vastly greater than might be explained by any precursor sample impurity, implying that interconversion of ethynylbenzyl motifs is feasible in energetic environments such as plasmas and flames, wherein resonance-stabilized radicals are persistent.

Electronic Spectroscopy of cis- and trans-meta-Vinylbenzyl Radicals.

The D0(2A″)-D1(2A″) electronic transition of resonance-stabilized radical C9H9 isomers cis- and trans-meta-vinylbenzyl (MVB) has been investigated using resonant two-color two-photon ionization (R2C2PI) and laser-induced fluorescence. The radicals were produced in a discharge of m-vinyltoluene diluted in Ar and probed under jet-cooled conditions. The origin bands of the cis and trans conformers are at 19 037 and 18 939 cm-1, respectively. Adiabatic ionization energies near 7.17 eV were determined for both conformers from two-color ion-yield scans. Dispersed fluorescence (DF) was used to conclusively identify the cis-conformer: ground-state cis-MVB eigenvalues calculated for a Fourier series fit of a computed vinyl torsion potential are in excellent agreement with torsional transitions in the 19 037 cm-1 DF spectrum. R2C2PI features arising from cis- or trans-MVB were distinguished by optical-optical hole-burning spectroscopy and vibronic assignments were made with guidance from density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. There is a notable absence of mirror symmetry between excitation and emission spectra for several totally symmetric modes, whereby modes that are conspicuous in emission are nearly absent in excitation, and vice versa. This effect is largely ascribed to interference between Franck-Condon and Herzberg-Teller contributions to the electronic transition moment, and its pervasiveness a consequence of the low symmetry (Cs) of the molecule, which permits intensity borrowing from several relatively bright electronic states of A″ symmetry.

Design and characterization of a velocity-map imaging apparatus for low-energy photo-ion spectroscopy using magneto-optically trapped atoms.

We present a velocity-map imaging (VMI) apparatus coupled with a magneto-optical trap (MOT) of 87Rb atoms designed for low-energy photo-ion spectroscopy. The VMI-electrode geometry uses a three-electrode configuration, and the focusing electric field is optimized based on systematic simulations of relatively low-energy ions. To calibrate the apparatus, we use resonant two-color two-photon ionization of rubidium atoms as Doppler-selected ions. This VMI system provides an accuracy of 0.15m/s and a resolution of 7.5m/s for photoions with speeds below 100m/s. Finally, details of the design, construction, and testing of the VMI-MOT system are presented.

Two-Color Resonant Two-Photon Mass-Analyzed Threshold Ionization of 2,4-Difluoroanisole and the Additivity Relation of Ionization Energy.

We used the two-color resonant two-photon ionization and mass-analyzed threshold ionization spectroscopic techniques to record the vibronic, photoionization efficiency, and cation spectra of 2,4-difluoroanisole. The cation spectra were obtained through ionization via seven intermediate vibronic states, which involved out-of-plane ring-F, in-plane ring-F, and ring-OCH3 bending vibrations as well as in-plane ring deformation vibrations. The band origin of the S1 ← S0 electronic transition of 2,4-difluoroanisole appeared at 35 556 ± 2 cm-1, and the adiabatic ionization energy was determined to be 67 568 ± 5 cm-1. The experimental data provided information on the active vibrations of aforementioned compound in the electronically excited state S1 and ground cationic state D0. A comparison of the experimental data obtained for 2,4-difluoroanisole in this study with the data obtained for other fluorine-substituted benzenes in previous studies indicated that the nature, location, and number of substituents influence electronic transition energy, ionization energy, and molecular vibration. In addition, a simple additivity relation might exist for predicting the ionization energy of multiply substituted benzene derivatives.

Experimental and theoretical study on p-chlorofluorobenzene in the S, S1 and D states

The geometric structures and vibration frequencies of para-chlorofluorobenzene (p-ClFPh) in the first excited state of neutral and ground state of cation were investigated by resonance-enhanced multiphoton ionization and slow electron velocity-map imaging. The infrared spectrum of S0 state and absorption spectrum for S1 ← S0 transition in p-ClFPh were also recorded. Based on the one-color resonant two-photon ionization spectrum and two-color resonant two-photon ionization spectrum, we obtained the adiabatic excited-state energy of p-ClFPh as 36302±4 cm−1. In the two-color resonant two-photon ionization slow electron velocity-map imagin spectra, the accurate adiabatic ionization potential of p-ClFPh was extrapolated as 72937±8 cm−1 via threshold ionization measurement. In addition, Franck-Condon simulation was performed to help us confidently ascertain the main vibrational modes in the S1 and D0 states. Furthermore, the mixing of vibrational modes between S0 → S1 and S1 → D0 has been analyzed.

Probing cooperativity in C-H⋯N and C-H⋯π interactions: Dissociation energies of aniline⋯(CH4)n (n = 1, 2) van der Waals complexes from resonant ionization and velocity mapped ion imaging measurements.

Recent studies of the weakly bound anisole⋯CH4 complex found a dual mode of binding, featuring both C/H⋯π and C/H⋯O noncovalent interactions. In this work, we examine the dissociation energies of related aniline⋯(CH4)n (n = 1, 2) van der Waals clusters, where both C/H⋯π and C/H⋯N interactions are possible. Using a combination of theory and experiments that include mass-selected two-color resonant two-photon ionization spectroscopy, two-color appearance potential (2CAP) measurements, and velocity-mapped ion imaging (VMI), we derive the dissociation energies of both complexes in the ground (S0), excited (S1), and cation radical (D0) states. As the amide group is non-planar in the ground state, the optimized ground state geometry of the aniline⋯CH4 1:1 complex shows two isomers, each with the methane positioned above the aniline ring. The observed redshift of the electronic origin from the aniline monomer is consistent with TDDFT calculations for the more stable isomer, where the methane sits on the same face as the amino hydrogens. The dissociation energies of the 1:1 complex, obtained from 2CAP measurements, are in good agreement with the calculated theoretical values from selected density functional theory methods. VMI data for the 1:1 complex gave a binding energy value overestimated by ∼179 cm-1 when compared to the 2CAP results, indicating that dissociative ionization selectively populates an excited vibrational level of the aniline cation radical. Given that the electron donating ability of aromatic substituents trends as -NH2 > -OCH3 > -CH3, it is noteworthy that the strength of methane binding also trends in this order, as found by experiment (dissociation energies in kJ/mol: 6.6 > 5.8 > 4.5) and predicted by theory (PBE0-D3/def2-QZVPPD, in kJ/mol: 6.9 > 6.0 > 5.0). For the 1:2 complex of aniline and methane, calculations predict that the more stable conformer is the one where the two methane molecules lie on opposite faces of the ring, consistent with the observed redshift of the electronic origin. Unlike the anisole-methane 1:2 complex, which shows an enhanced dissociation energy for the loss of one methane in comparison with the 1:1 complex, here, we find that the energy required to remove one methane from the ground state aniline-methane 1:2 complex is smaller than that of the 1:1 complex, consistent with theoretical expectations.

Conformer Specific Ultraviolet and Infrared Detection of Nicotine in the Vapor Phase.

A gas-phase electronic spectrum of nicotine in a supersonic expansion has been recorded using two-color resonant two-photon ionization spectroscopy. Efficient photoionization was achievable only via the pyridine chromophore owing to poor Franck-Condon overlap in the N-methylpyrrolidine moiety. Two conformers of nicotine have been characterized and assigned by infrared-ultraviolet (IR-UV) ion depletion and IR-UV hole-burning spectroscopy, in combination with quantum chemical techniques. Trans-A with nitrogen atoms further apart is more stable by 2 kJ mol-1 and the most populated conformer in the supersonic jet, owing this stability to a stronger inter-ring CH···N hydrogen bond than the trans-B counterpart.

Excited-state vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-ethylcytosine.

The S1 excited-state lifetime of jet-cooled 1-ethylcytosine (1ECyt) is ∼1 ns, one of the longest lifetimes for cytosine derivatives to date. Here, we analyze its S0 → S1 vibronic spectrum using two-color resonant two-photon ionization and UV/UV holeburning spectroscopy. Compared to cytosine and 1-methylcytosine, the S0 → S1 spectrum of 1ECyt shows a progression in the out-of-plane "butterfly" mode ν1 ', identified by spin-component scaled-second-order coupled-cluster method ab initio calculations. We also report time-resolved S1 state nonradiative dynamics at ∼20 ps resolution by the pump/delayed ionization technique. The S1 lifetime increases with the number of ν1 ' quanta from τ = 930 ps at v1 '=0 to 1030 ps at v1 '=2, decreasing to 14 ps at 710 cm-1 vibrational energy. We measured the rate constants for S1 ⇝ S0 internal conversion and S1 ⇝ T1 intersystem crossing (ISC): At the v' = 0 level, kIC is 8 × 108 s-1 or three times smaller than 1-methylcytosine. The ISC rate constant from v' = 0 to the T1(3ππ*) state is kISC = 2.4 × 108 s-1, 10 times smaller than the ISC rate constants of cytosine, but similar to that of 1-methylcytosine. Based on the calculated S1(1ππ*) state radiative lifetime τrad = 12 ns, the fluorescence quantum yield of 1ECyt is Φfl ∼ 7% and the intersystem crossing yield is ΦISC ∼ 20%. We measured the adiabatic ionization energy of 1-ethylcytosine via excitation of the S1 state as 8.353 ± 0.008 eV, which is 0.38 eV lower than that of amino-keto cytosine. Measurement of the ionization energy of the long-lived T1(ππ*) state formed via ISC reveals that it lies 3.2-3.4 eV above the S0 ground state.

Cation spectra of p-chloroanisole and the heavy atom effect on ionization energy

Vibrationally resolved cation spectra of p-chloroanisole were recorded by ionizing via four vibronic levels through two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy techniques. The results indicated that the geometry and molecular vibrations 7a, 16b, and 6a of p-chloroanisole in the cationic ground D0 state resemble those in the electronically excited neutral S1 state. The precisely measured adiabatic ionization energy of this molecule was less than that of p-fluoroanisole by 325 cm−1. This can be labeled as a heavy atom effect, which is similar to that observed for para halogen-substituted phenol and aniline.

Cation Vibrations of 1-Methylnaphthalene and 2-Methylnaphthalene through Mass-Analyzed Threshold Ionization Spectroscopy.

Vibrationally resolved cation spectra of 1-methylnaphthalene (1MN) and 2-methylnaphthalene (2MN) were obtained using the two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy. MATI spectra were obtained through ionization via several intermediate vibronic levels. Due to hindrance effect, no spectral features related to methyl torsion were observed in the MATI spectra of 1MN. By contrast, most of the in-plane ring deformation vibrations of the 2MN cation were found to couple with methyl torsion because of its small internal rotational barrier. These experimental findings were well supported by our theoretical calculations.

C-H/π and C-H-O Interactions in Concert: A Study of the Anisole-Methane Complex using Resonant Ionization and Velocity Mapped Ion Imaging.

Noncovalent forces such as hydrogen bonding, halogen bonding, π-π stacking, and C-H/π and C-H/O interactions hold the key to such chemical processes as protein folding, molecular self-assembly, and drug-substrate interactions. Invaluable insight into the nature and strength of these forces continues to come from the study of isolated molecular clusters. In this work, we report on a study of the isolated anisole-methane complex, where both C-H/π and C-H/O interactions are possible, using a combination of theory and experiments that include mass-selected two-color resonant two-photon ionization spectroscopy, two-color appearance potential (2CAP) measurements, and velocity mapped ion imaging (VMI). Using 2CAP and VMI, we derive the binding energies of the complex in ground, excited, and cation radical states. The experimental values from the two methods are in excellent agreement, and they are compared with selected theoretical values calculated using density functional theory and ab initio methods. The optimized ground-state cluster geometry, which is consistent with the experimental observations, shows methane sitting above the ring, interacting with anisole via both C-H/π and C-H/O interactions, and this dual mode of interaction is reflected in a larger ground-state binding energy as compared with the prototypical benzene-methane system.

Transition from Water Wires to Bifurcated H-Bond Networks in 2-Pyridone·(H2O) n, n = 1-4 Clusters.

Mass-selective two-color resonant two-photon ionization (2C-R2PI), UV/UV hole-burning, and infrared (IR) depletion spectra of supersonic jet-cooled 2-pyridone·(H2O) n clusters with n = 1-4 have been measured to investigate the local hydration patterns around 2-pyridone (2PY) as a function of cluster size. As shown by others, the IR frequencies of the OH and NH stretches of the n = 1, 2 clusters are characteristic of water wires stretching from the NH to the C═O group of 2PY. We identify two isomers (3A and 3B) of the n = 3 cluster in the 2C-R2PI spectrum and separate them by IR/UV and UV/UV hole-burning techniques. Isomer 3A exhibits a three-membered water wire, extending the n = 1, 2 structural motif. Isomer 3B exhibits bifurcated water wires with the first H2O donating to two waters that form H-bonds to the C═O group. This increases the H-bond strength between the NH group of 2PY and the proximal H2O molecule, lowering the NH stretch to ∼2800 cm-1. The n = 4 cluster is also bifurcated with two water wires between the bifurcating H2O and the C═O group. The cluster-selective IR spectra are complemented with density-functional calculations using the PW91, B3LYP, B97-D, and M06-2X functionals, where the latter two include long-range dispersive interactions, and with the ab initio correlated SCS-CC2 method. The calculated IR spectra provide firm assignments of the structures of the n = 1-4 cluster structures and allow us to understand the evolution of individual H-bond strengths with increasing cluster size.

- stacking vs. C-H/ interaction: Excimer formation and charge resonance stabilization in van der Waals clusters of 9,9'-dimethylfluorene.

Studies of exciton and hole stabilization in multichromophoric systems underpin our understanding of electron transfer and transport in materials and biomolecules. The simplest model systems are dimeric, and recently we compared the gas-phase spectroscopy and dynamics of van der Waals dimers of fluorene, 9-methylfluorene (MF), and 9,9'-dimethylfluorene (F1) to assess how sterically controlled facial encumbrance modulates the dynamics of excimer formation and charge resonance stabilization (CRS). Dimers of fluorene and MF show only excimer emission upon electronic excitation, and significant CRS as evidenced in a reduced ionization potential for the dimer relative the monomer. By contrast, the dimer of F1 shows no excimeric emission, rather structured emission from the locally excited state of a tilted (non π-stacked) dimer, evidencing the importance of C-H/π interactions and increased steric constraints that restrict a cofacial approach. In this work, we report our full results on van der Waals clusters of F1, using a combination of theory and experiments that include laser-induced fluorescence, mass-selected two-color resonant two-photon ionization spectroscopy, and two-color appearance potential measurements. We use the latter to derive the binding energies of the F1 dimer in ground, excited, and cation radical states. Our results are compared with van der Waals and covalently linked clusters of fluorene to assess both the relative strength of π-stacking and C-H/π interactions in polyaromatic assemblies and the role of π-stacking in excimer formation and CRS.

N–H···S Interaction Continues To Be an Enigma: Experimental and Computational Investigations of Hydrogen-Bonded Complexes of Benzimidazole with Thioethers

The N-H···S hydrogen bond, even though classified as an unconventional hydrogen bond, is found to bear important structural implications on protein structure and folding. In this article, we report a gas-phase study of the N-H···S hydrogen bond between the model compounds of histidine (benzimidazole, denoted BIM) and methionine (dimethyl sulfide, diethyl sulfide, and tetrahydrothiophene, denoted Me2S, Et2S, and THT, respectively). A combination of laser spectroscopic methods such as laser-induced fluorescence (LIF), two-color resonant two-photon ionization (2cR2PI), and fluorescence depletion by infrared spectroscopy (FDIR) is used in conjunction with DFT and ab initio calculations to characterize the nature of this prevalent H-bonding interaction in simple bimolecular complexes. A single conformer was found to exist for the BIM-Me2S complex, whereas the BIM-Et2S and BIM-THT complexes showed the presence of three and two conformers, respectively. These conformers were characterized on the basis of IR spectroscopic results and electronic structure calculations. Quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO), and energy decomposition (NEDA) analyses were performed to investigate the nature of the N-H···S H-bond. Comparison of the results with the N-H···O type of interactions in BIM and indole revealed that the strength of the N-H···S H-bond is similar to N-H···O in these binary gas-phase complexes.

Cofacially Arrayed Polyfluorenes: Spontaneous Formation of π-Stacked Assemblies in the Gas Phase.

Understanding geometrical and size dependencies of through-space charge delocalization in multichromophoric systems is critical to model electron transfer and transport in materials and biomolecules. In this work, we examine the size evolution of hole delocalization in van der Waals clusters of fluorene (i.e., (F)n), where a range of geometries are possible, reflecting both π-stacking and C-H/π interactions. Using mass-selected two-color resonant two-photon ionization spectroscopy (2CR2PI), we measure electronic spectra and vertical ionization potentials (IPs) in the gas phase. Results are compared with model covalently linked assemblies (denoted Fn), exhibiting a sterically enforced cofacial (i.e., π-stacked) orientation of chromophores. For both systems, an inverse size dependence (i.e., 1/n) of IP vs cluster size is found. Surprisingly, the values for the two sets fall on the same line! This trend is examined via theory, which emphasizes the important role of π-stacking, and its geometrical dependencies, in the process of hole delocalization in multichromophoric assemblies.

Elevation of the Energy Threshold for Isomerization of 5-Hydroxyindole-(tert-butyl alcohol)1 Cluster Cations.

Isomerization between two hydrogen-bonded (H-bonded) isomers of 5-hydroxyindole-(tert-butyl alcohol)1 cluster cations ([5HI-(t-BuOH)1]+) was investigated in the gas phase. In the S0 state, jet-cooled 5HI-(t-BuOH)1 has two structural isomers, 5HI(OH)-(t-BuOH)1 and 5HI(NH)-(t-BuOH)1, in which the t-BuOH molecule is bound to the OH or the NH group of 5HI. The IR photodissociation spectrum of [5HI-(t-BuOH)1]+ generated by two-color resonant two-photon ionization (2C-R2PI) via the S1-S0 origin of 5HI(NH)-(t-BuOH)1 provided evidence of both [5HI(OH)-(t-BuOH)1]+ and [5HI(NH)-(t-BuOH)1]+ coexisting in the D0 state, indicating that [5HI(NH)-(t-BuOH)1]+ isomerizes to [5HI(OH)-(t-BuOH)1]+ after 2C-R2PI of 5HI(NH)-(t-BuOH)1. The lower limit of the energy threshold for the isomerization of [5HI(NH)-(t-BuOH)1]+ to [5HI(OH)-(t-BuOH)1]+ was experimentally determined to be 3362 ± 30 cm-1, and the corresponding energy threshold for the isomerization of [5HI(NH)-(H2O)1]+ to [5HI(OH)-(H2O)1]+ has been reported to be 2127 ± 30 cm-1. Thus, the energy threshold for the isomerization is elevated by at least 1200 cm-1 when the solvent molecule changes from H2O to t-BuOH. The elevation of the energy threshold is explained by the difference in the stabilization energies of [5HI-(t-BuOH)1]+ and [5HI-(H2O)1]+ in the initial and transition states owing to the larger proton affinity of t-BuOH than H2O.

Effect of Facial Encumbrance on Excimer Formation and Charge Resonance Stabilization in Model Bichromophoric Assemblies

Excimer formation and charge resonance stabilization in covalently linked bichromophoric systems with flexible spacers are important processes relevant to biochemistry and functional materials. Requiring a π-stacked cofacial arrangement of a pair of aromatic molecules at a van der Waals contact, the underlying geometrical reorganization that accompanies these events continues to be debated. Here we use a variety of methods including two-color resonant two-photon ionization spectroscopy (2CR2PI), ion yield measurements, hole-burning spectroscopy (HB), and laser-induced fluorescence (LIF) excitation and emission spectroscopy to compare the gas-phase spectroscopy and dynamics of the van der Waals dimers of fluorene, 9-methylfluorene (MF), and 9,9′-dimethylfluorene (F1). The goal of this work is to probe the influence of methyl substitution on the dynamics of excimer formation and charge resonance (CR) stabilization. The fluorene dimer, (F)2, displays lifetime broadened electronic spectra and the dominance of...

Planarizing cytosine: The S1 state structure, vibrations, and nonradiative dynamics of jet-cooled 5,6-trimethylenecytosine.

We measure the S0 → S1 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 ∼500 cm-1 above its 000 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 S0 → S1 geometry changes in detail. The TMCyt S1 state out-of-plane vibrations ν1', ν3', and ν5' lie below 420 cm-1, and the in-plane ν11', ν12', and ν23' vibrational fundamentals appear at 450, 470, and 944 cm-1. S0 → S1 vibronic simulations based on SCS-CC2 calculations agree well with experiment if the calculated ν1', ν3', and ν5' frequencies are reduced by a factor of 2-3. MS-CASPT2 calculations predict that the ethylene-type S1 ⇝ S0 conical intersection (CI) increases from +366 cm-1 in cytosine to >6000 cm-1 in TMCyt, explaining the long lifetime and extended S0 → S1 spectrum. The lowest-energy S1 ⇝ S0 CI of TMCyt is the "amino out-of-plane" (OPX) intersection, calculated at +4190 cm-1. The experimental S1 ⇝ S0 internal conversion rate constant at the S1(v'=0) level is kIC=0.98-2.2⋅108 s-1, which is ∼10 times smaller than in 1-methylcytosine and cytosine. The S1(v'=0) level relaxes into the T1(3ππ*) state by intersystem crossing with kISC=0.41-1.6⋅108 s-1. The T1 state energy is measured to lie 24 580±560 cm-1 above the S0 state. The S1(v'=0) lifetime is τ=2.9 ns, resulting in an estimated fluorescence quantum yield of Φfl=24%. 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 8.114±0.002 eV.