Time-resolved conformational dynamics in hydrocarbon chains

Observation of reversible conformational interconversion accompanied by 3p internal conversions in Rydberg-excited N,N-dimethylethylamine

Conformational dynamics has been well observed in the 3s Rydberg state of amines, whereas its observation in higher-energy, non-equilibrium 3p Rydberg states is very rare, especially for a reversible conformational transition that could compete with other non-adiabatic transitions. Herein, we report the observation of a reversible conformational interconversion phenomenon in the 3p Rydberg excited-state dynamics of N,N-dimethylethylamine (DMEA). Upon electronic excitation, a forward and backward interconversion between the initially prepared 3p_l and 3p_h conformers accompanied by 3p internal conversions occurs, resulting in a 3p_l/3p_h equilibrium ratio of 61 %/39 % within ∼1.5 ps. The ensuing parallel internal conversions from the 3p_l to 3s_l and 3p_h to 3s_h deposit about 1.80 eV of vibrational energy into the 3s state, enabling a fast conformational interconversion between the 3s_h and 3s_l conformers to proceed within ∼2.0 ps. The final 3s_l/3s_h equilibrium ratio was determined to be 76 %/24 %. This work presents a real-time observation of the entire conformational interconversion process initiating from the higher-energy 3p states and finally reaching an equilibrium on the lower-energy 3s state.

Femtosecond Time-Resolved Observation of Relaxation and Wave Packet Dynamics of the S1 State in Electronically Excited o-Fluoroaniline

Quantum beat frequency is the basis for understanding interference effects and vibrational wave packet dynamics and has important applications. Using femtosecond time-resolved mass spectrometry and femtosecond time-resolved photoelectron image combined with theoretical calculations, we study the electronic excited-state relaxation of o-fluoraniline molecule and the time-dependent evolution of vibrational wave packets between different eigenstates. After the molecule absorbs a photon of 288.3 nm and is excited to the S₁ state, intramolecular vibrational redistribution first occurs on the time scale τ₁ = 349 fs, and then the transition to the triplet state occurs through the intersystem crossing on the time scale τ₂ = 583 ps, and finally, the triplet state occurs decays slowly through the time scale τ₃ = 2074 ps. We find the intramolecular vibrational redistribution is caused by the 0⁰, 10b¹ and 16a¹ vibrational modes of the S_l state origin. That is, the 288.3 nm femtosecond laser excites the molecule to the S₁ state, and the continuous flow of the vibrational wave packet prepares a coherent superposition state of three vibrational modes. Through extracting the oscillation of different peak intensities in the photoelectron spectrum, we observe reversible changes caused by mutual interference of the S₁ 0⁰, S₁ 10b¹ and S₁ 16a¹ states when the wave packets flow. When the pump pulse is 280 nm, the beat frequency disappears completely. This is explained in terms of increases in the vibrational field density and characteristic period of oscillation, and statistical averaging makes the quantum effect smooth and indistinguishable. In addition, the Rydberg component of the S₁ state is more clearly resolved by combining experiment and theory.

Ultrafast conformational dynamics of Rydberg-excited N-methyl piperidine

We have observed the ultrafast conformational dynamics of electronically excited N-methyl piperidine (NMP) using time-resolved Rydberg fingerprint spectroscopy. Optical excitation at various wavelengths ranging from 212 nm to 229 nm leads to the 3s or 3p Rydberg states and induces coherent oscillatory motions with periods of about 700 fs. These coherent motions survive the internal conversion from 3p to 3s but then dephase on a time scale of a few oscillations. Intramolecular vibrational energy redistribution on a picosecond time scale leads to an equilibrium between two conformeric structures that are separated in binding energy by 0.09 eV. Model calculations using the DFT-SIC method are in excellent agreement with the experiments and identify the conformers as the chair and twist structures of NMP. The analysis of the equilibrium parameters at long time delays as a function of excitation wavelength allows for the extraction of thermodynamic parameters for the conformeric transformation. We derive an enthalpy of the chair to twist reaction in the 3s excited state of 62 meV with an entropy of 19.70 J mol^-1 K^-1. An activation energy of 276 meV is also obtained with a kinetic model.

Water-Soluble Visible Light Sensitive Photoinitiating System Based on Charge Transfer Complexes for the 3D Printing of Hydrogels

The development of visible-light 3D printing technology by using water-soluble initiating systems has attracted widespread attention due to their potential applications in the manufacture of hydrogels. Besides, at present, the preparation of water-soluble photoinitiators suitable for visible light irradiation (such as LEDs) still remains a challenge. Therefore, this work is devoted to developing water-soluble photoinitiators (PI)/photoinitiating systems (PIS) upon irradiation with a LED @ 405 nm. In detail, a new water-slightly-soluble chalcone derivative dye [(E)-3-(4-(dimethylamino) phenyl)-1-(4-(2-(2-(2-methoxyethoxy) ethoxy) ethoxy) phenyl) prop-2-en-1-one] was synthesized here and used as a PI with a water-soluble coinitiator, i.e., triethanolamine (TEA) which was also used as an electron donor. When combined together, a charge transfer complex (CTC) formed immediately which exhibited excellent initiating ability for the free radical photopolymerization of poly(ethyleneglycol)diacrylate (PEG-DA). In light of the powerful CTC effect, the [dye-TEA] CTC could not only exhibit enhanced water solubility and mechanical properties but could also be effectively applied for 3D printing. This CTC system is environmentally friendly and cost-saving which demonstrates a great potential to prepare hydrogels via photopolymerization.

How the methyl group position influences the ultrafast deactivation in aromatic radicals

Excited xylyl (methyl–benzyl) radical isomers have been studied by femtosecond time-resolved photoelectron spectroscopy and mass spectrometry.

Observation of Structural Wavepacket Motion: The Umbrella Mode in Rydberg-Excited N-Methyl Morpholine

We have observed time-resolved, structural dynamics of a coherent vibrational wavepacket in Rydberg-excited N-methyl morpholine, a molecule with 48 internal degrees of freedom. The molecular structure was established by associating the time-dependent Rydberg electron binding energy, obtained from time-resolved photoionization-photoelectron spectroscopy, to the molecular structure using self-interaction corrected density functional calculations. Optical excitation at 226 nm launches an oscillatory wavepacket in the amine umbrella coordinate with a 650 fs period. Even though the Franck-Condon excitation is at an angle of 17°, the wavepacket settles into an oscillation between 4° and -10° within a fraction of a vibrational period and then dephases with a time constant of 750 fs.

Coherence in nonradiative transitions: internal conversion in Rydberg-excited N-methyl and N-ethyl morpholine

The survival of coherent wavepacket motion during internal conversions is observed in relatively large molecules, N-methyl morpholine and N-ethyl morpholine, where standard models imply fast decoherence in a statistical limit.

The effects of symmetry and rigidity on non-adiabatic dynamics in tertiary amines: a time-resolved photoelectron velocity-map imaging study of the cage-amine ABCO

The non-adiabatic relaxation dynamics of the tertiary cage-amine azabicyclo[2.2.2]octane (ABCO, also known as quinuclidine) have been investigated following 3p Rydberg excitation at 201 nm using femtosecond time-resolved photoelectron imaging (TRPEI). The aim of the study was to investigate the influence of the rigid and symmetric cage structure found in ABCO on the general non-adiabatic relaxation processes commonly seen in other tertiary aliphatic amines (TAAs). Our data is compared with TRPEI results very recently obtained for several structurally less rigid TAA systems [J. O. F. Thompson et al., Chem. Sci., 2016, 7, 1826-1839] and helps to confirm many of the previously reported findings. The experimental results for ABCO in the short-time (<1 ps) regime strongly support earlier conclusions suggesting that planarization about the N-atom is not a prerequisite for efficient 3p-3s internal conversion. Additionally, individual photoelectron peaks within our ABCO data show no temporal shifts in energy. As confirmed by our supporting quantum mechanical calculations, this demonstrates that neither internal conversion within the 3p manifold or significant conformational re-organization are possible in the ABCO system. This result therefore lends strong additional support to the active presence of such dynamical effects in other, less conformationally restricted TAA species, where photoelectron peak shifts are commonly observed. Finally, the extremely long (>1 ns) 3s Rydberg state lifetime seen in ABCO (relative to other TAA systems at similar excitation energies) serves to illustrate the large influence of symmetry and conformational rigidity on intramolecular vibrational redistribution processes previously implicated in mediating this aspect of the overall relaxation dynamics.

Charge transfer and ultrafast nuclear motions: the complex structural dynamics of an electronically excited triamine

Three ionization centers make 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC) an interesting model system to study intramolecular charge transfer (CT). Because the molecule assumes a Cs symmetric, axial-equatorial-equatorial (aee) conformation in the ground state, there are two distinct types of the nitrogen atoms. We discovered that either nitrogen atom can be ionized independently so that two molecular cations exist with different (localized) charge distributions in the Franck-Condon region. The initially localized charge can delocalize via CT, provided the molecule acquires a suitable structural geometry. These proper structures are all found to have a common structural motif that supports CT via through-space-interaction. The structural dynamics and the CT process in Rydberg-excited TMTAC, where the molecular ion core closely resembles the ion, were probed by time-resolved Rydberg fingerprint spectroscopy. When TMTAC is excited at 230 nm to the Franck-Condon region of the 3s Rydberg state, the two types of nitrogen atom Rydberg chromophores give rise to distinct binding energy peaks. The sequential molecular responses that follow the Rydberg excitation manifest themselves as time-dependent changes of the binding energy and are observed by ionization at 404 nm. A fast transition with 103 fs time constant was attributed to a motion that leads to a local minimum of the charge-localized state on the Rydberg potential energy surface. Because a large amount of energy is deposited into the vibrational manifolds, the molecule continues to sample the potential energy surface and eventually reaches a dynamic equilibrium between charge-localized and charge-delocalized states. The forward and backward time constants were determined to be 1.02 ps and 4.09 ps, respectively. The binding energy spectrum also reveals the existence of an equilibrium among several charge-delocalized states. Quantum chemical calculations were carried out to find the stable minima of the ground state and the ion state. The binding energies of the Franck-Condon structures and the relaxed ion structures were calculated using the Perdew-Zunger self-interaction corrected DFT (PZ-SIC) method to assign the spectra at time zero and at equilibrium, respectively.

Ultrafast relaxation dynamics of electronically excited piperidine: ionization signatures of Rydberg/valence evolution

Time-resolved photoelectron spectroscopy reveals distinct ionization signatures of Rydberg-to-valence state evolution in the secondary aliphatic amine piperidine.

The role of novel Rydberg-valence behaviour in the non-adiabatic dynamics of tertiary aliphatic amines

Time-resolved photoelectron imaging was used to study non-adiabatic relaxation dynamics in N,N-dimethylisopropylamine, N,N-dimethylpropylamine and N-methylpyrrolidine following excitation at 200 nm. This series of tertiary aliphatic amines are all of similar chemical makeup, but exhibit differences in their structure - being branched, straight-chain and cyclic, respectively. Our experimental investigation, supported by extensive theoretical calculations, provides considerable new insight into the nature of the internal conversion processes that mediate dynamical evolution between electronic states of predominantly Rydberg character in this important class of model photochemical systems. In particular, the angle-resolved data afforded by the imaging approach (something not previously reported for tertiary aliphatic amines) offers novel and highly-detailed mechanistic information about the overall relaxation pathway. Strikingly, both the experimental and theoretical findings suggest that a critical factor driving the non-adiabatic dynamics is the evolution of valence character along an N-C stretching coordinate within a member of the 3p manifold. This is in stark contrast to primary and secondary amines, as well as many other small hetero-atom containing organic species, where evolution of valence character within the 3s state is now a well-established phenomenon implicated in mediating ultrafast non-adiabatic photochemistry.

Ultrafast structural pathway of charge transfer in n,n,n',n'-tetramethylethylenediamine

We have explored the ultrafast molecular structural dynamics associated with charge transfer in N,N,N',N'-tetramethylethylenediamine using Rydberg fingerprint spectroscopy in conjunction with self-interaction corrected density functional theory. Excitation at 239 nm prepares the molecule in the Franck-Condon region of the 3s state with the charge localized on one of the two amine groups. As seen from the time-dependent Rydberg electron binding energies, the pathway of the rapidly ensuing dynamics leads through several structurally distinct conformers with various degrees of charge localization before reaching the fully charge-delocalized structure on a picosecond time scale. At several steps along the reaction path, the transient structures are identified through a comparison of the spectroscopically observed binding energies with computed values. The molecular structure is seen to evolve dynamically from an initially folded conformer to the stretched form that supports charge delocalization before an equilibrium sets in with forward and backward time constants of 1.19 (0.14) and 2.61 (0.31) ps, respectively. A coherent wavepacket motion in the charge-localized state with a period of 270 (17) fs and damping of 430 (260) fs is observed and tentatively assigned to the nitrogen umbrella motion. The damping time constant indicates the rate of the energy flow into other vibrations that are not activated by the optical excitation.

Ultrafast structural dynamics in Rydberg excited N,N,N′,N′-tetramethylethylenediamine: conformation dependent electron lone pair interaction and charge delocalization

Time-resolved Rydberg fingerprint spectroscopy and quantum calculations reveal the structure dependent electron lone pair interaction and charge delocalization in real time.

Toward structural femtosecond chemical dynamics: imaging chemistry in space and time

We aim to observe a chemical reaction in real time using gas-phase X-ray diffraction. In our initial experiment at the Linac Coherent Light Source (LCLS), we investigated the model system 1,3-cyclohexadiene (CHD) at very low vapor pressures. This reaction serves as a benchmark for numerous transformations in organic synthesis and natural product biology. Excitation of CHD by an ultraviolet optical pulse initiates an electrocyclic reaction that transforms the closed ring system into the open-chain structure of 1,3,5-hexatriene. We describe technical points of the experimental method and present first results. We also outline an approach to analyze the data involving nonlinear least-square optimization routines that match the experimental observations with predicted diffraction patterns calculated from trajectories for nonadiabatic vibronic wave packets.

Ultrafast structural and isomerization dynamics in the Rydberg-exited quadricyclane: norbornadiene system

The quadricyclane-norbornadiene system is an important model for the isomerization dynamics between highly strained molecules. In a breakthrough observation for a polyatomic molecular system of that complexity, we follow the photoionization from Rydberg states in the time-domain to derive a measure for the time-dependent structural dynamics and the time-evolving structural dispersion even while the molecule is crossing electronic surfaces. The photoexcitation to the 3s and 3p Rydberg states deposits significant amounts of energy into vibrational motions. We observe the formation and evolution of the vibrational wavepacket on the Rydberg surface and the internal conversion from the 3p Rydberg states to the 3s state. In that state, quadricyclane isomerizes to norbornadiene with a time constant of τ(2) = 136(45) fs. The lifetime of the 3p Rydberg state in quadricyclane is τ(1) = 320(31) and the lifetime of the 3s Rydberg state in norbornadiene is τ(3) = 394(32).

Far-UV photochemical bond cleavage of n-amyl nitrite: bypassing a repulsive surface

We have investigated the deep-UV photoinduced, homolytic bond cleavage of amyl nitrite to form NO and pentoxy radicals. One-color multiphoton ionization with ultrashort laser pulses through the S(2) state resonance gives rise to photoelectron spectra that reflect ionization from the S(1) state. Time-resolved pump-probe photoionization measurements show that upon excitation at 207 nm, the generation of NO in the v = 2 state is delayed, with a rise time of 283 (16) fs. The time-resolved mass spectrum shows the NO to be expelled with a kinetic energy of 1.0 eV, which is consistent with dissociation on the S(1) state potential energy surface. Combined, these observations show that the first step of the dissociation reaction involves an internal conversion from the S(2) to the S(1) state, which is followed by the ejection of the NO radical on the predissociative S(1) state potential energy surface.

Standoff detection of large organic molecules using Rydberg fingerprint spectroscopy and microwave Rayleigh scattering

We present a technique for nonintrusive and standoff detection of large organic molecules using coherent microwave Rayleigh scattering from plasma produced by structure sensitive photoionization through Rydberg states. We test the method on 1,4-diazobicyclooctane. Transitions between the 3s Rydberg state and higher lying Rydberg states are probed using two-color photoionization with 266 nm photons and photons in the range of 460-2400 nm. Photoionization is detected using microwave radiation, which is scattered by the unbounded electrons. Highly resolved Rydberg spectra are acquired in vacuum and in air.

Electronic spectroscopy and ultrafast energy relaxation pathways in the lowest Rydberg States of trimethylamine

Resonance-enhanced multiphoton ionization photoelectron spectroscopy has been applied to study the electronic spectroscopy and relaxation pathways among the 3p and 3s Rydberg states of trimethylamine. The experiments used femtosecond and picosecond duration laser pulses at wavelengths of 416, 266, and 208 nm and employed two-photon and three-photon ionization schemes. The binding energy of the 3s Rydberg state was found to be 3.087 +/- 0.005 eV. The degenerate 3p x, y states have binding energies of 2.251 +/- 0.005 eV, and 3p z is at 2.204 +/- 0.005 eV. Using picosecond and femtosecond time-resolved experiments we spectrally and temporally resolved an intricate sequence of energy relaxation pathways leading from the 3p states to the 3s state. With excitation at 5.96 eV, trimethylamine is found to decay from the 3p z state to 3p x, y in 539 fs. The decay to 3s from all the 3p states takes place with a 2.9 ps time constant. On these time scales, trimethylamine does not fragment at the given internal energies, which range from 0.42 to 1.54 eV depending on the excitation wavelength and electronic state.