Triplet Mediated C-N Dissociation versus Internal Conversion in Electronically Excited N-Methylpyrrole

Ultrafast decay dynamics of electronically excited 2-ethylpyrrole

The excited-state decay dynamics of 2-ethylpyrrole following UV excitation in the wavelength range of 254.8-218.0 nm is investigated in detail using the femtosecond time-resolved photoelectron imaging method. The time-resolved photoelectron spectra and photoelectron angular distributions at all pump wavelengths are carefully analysed and the following picture is derived: at the longest pump wavelengths (254.8, 248.3 and 246.1 nm), 2-ethylpyrrole is excited to the S₁(¹πσ*) state having a lifetime of about 50 fs. At 248.3, 246.1 and 237.4 nm, another excited state of Rydberg character is excited. The lifetime of this state is ∼570 fs at 237.4 nm and becomes slightly longer at other two pump wavelengths. At the shortest pump wavelengths (230.8 and 218.0 nm), 2-ethylpyrrole is excited to a state which is tentatively assigned to the 1¹ππ* state, having a lifetime of 75 ± 15 and 48 ± 10 fs for the longer and shorter pump wavelengths, respectively. Internal conversion to the S₁(¹πσ*) state might be one of the decay mechanisms of the 1¹ππ* state.

Electronic, vibrational, and torsional couplings in N-methylpyrrole: Ground, first excited, and cation states

The electronic spectrum associated with the S₁ ← S₀ (ùA₂←X̃¹A₁) one-photon transition of jet-cooled N-methylpyrrole is investigated using laser-induced fluorescence (LIF) and (1 + 1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy; in addition, the (2 + 2) REMPI spectrum is considered. Assignment of the observed bands is achieved using a combination of dispersed fluorescence (DF), two-dimensional LIF (2D-LIF), zero-electron-kinetic energy (ZEKE) spectroscopy, and quantum chemical calculations. The spectroscopic studies project the levels of the S₁ state onto those of either the S₀ state, in DF and 2D-LIF spectroscopy, or the ground state cation (D₀ ⁺) state, in ZEKE spectroscopy. The assignments of the spectra provide information on the vibrational, vibration-torsion (vibtor), and torsional levels in those states and those of the S₁ levels. The spectra are indicative of vibronic (including torsional) interactions between the S₁ state and other excited electronic states, deduced both in terms of the vibrational activity observed and shifts from expected vibrational wavenumbers in the S₁ state, attributed to the resulting altered shape of the S₁ surface. Many of the ZEKE spectra are consistent with the largely Rydberg nature of the S₁ state near the Franck-Condon region; however, there is also some activity that is less straightforward to explain. Comments are made regarding the photodynamics of the S₁ state.

Role of Triplet States in the Photodynamics of Aniline

The dynamics of excited heteroaromatic molecules is a key to understanding the photoprotective properties of many biologically relevant chromophores that dissipate their excitation energy nonreactively and thereby prevent the detrimental effects of ultraviolet radiation. Despite their structural variability, most substituted aromatic compounds share a common feature of a repulsive ¹πσ* potential energy surface. This surface can lead to photoproducts, and it can also facilitate the population transfer back to the ground electronic state by means of a ¹πσ*/S₀ conical intersection. Here, we explore a hidden relaxation route involving the triplet electronic state of aniline, which has recently been discovered by means of time-selected photofragment translational spectroscopy [J. Chem. Phys. 2019, 151, 141101]. By using the recently available analytical gradients for multiconfiguration pair-density functional theory, it is now possible to locate the minimum-energy crossing points between states of different spin and therefore compute the intersystem crossing rates with a multireference method, rather than with the less reliable single-reference methods. Using such calculations, we demonstrate that the population loss of aniline in the T₁(³ππ*) state is dominated by C₆H₅NH₂ → C₆H₅NH· + H· dissociation, and we explain the long nonradiative lifetimes of the T₁(³ππ*) state at the excitation wavelengths of 294-264 nm.

Mode-specific excited-state dynamics of N-methylpyrrole

State-selective deactivation rates of N-methylpyrrole in the S₁ state have been measured by using the picosecond pump-probe method. The S₁ decay time leading to the N-CH₃ bond dissociation is found to be strongly mode-dependent as manifested in both S₁ decay and methyl-fragment growth dynamics. Time-resolved velocity-map ion images of the ˙CH₃ fragment, as far as the fragment of the Gaussian-shaped high kinetic energy distribution is concerned, suggest that the N-CH₃ cleavage reaction might occur through an intermediate. Sudden decrease of the S₁ lifetime at ∼700 cm^-1 above the S₁ origin is accompanied by the fragmentation of the Boltzmann-type low kinetic energy distribution. The appearance rate of this low-kinetic energy fragment turns out to be quite slow to give τ∼ 5 ns compared to the S₁ lifetime of ∼174 ps at the +806 cm^-1 band, for instance, confirming previous findings that the S₁ decay process starts to be overwhelmed by a new fast nonradiative transition in the corresponding excitation energy region. The lifetime at the S₁ origin accessed by the two-photon absorption is firstly measured to give τ∼ 8 ns. Using one and two photon absoption processes, a number of S₁ vibronic bands are identified to give mode-dependent lifetimes spanning an enormously wide temporal range of 8 ns-5 ps in the quite narrow excitation energy region of 0-1800 cm^-1 above the S₁ origin. Understanding of the N-methylpyrrole dynamics on multidimensional excited-state potential energy surfaces governing energy dissipating processes will get much benefit from our detailed mode-specific lifetime measurements.

Mechanism of the Chemiluminescent Reaction between Nitric Oxide and Ozone

The gas phase reaction of nitric oxide with ozone to give chemiluminescence is used extensively for detection of nitrogen oxides. The molecular mechanism of chemiluminescence in this reaction is not known. So far, the only chemiluminescent systems studied in depth are certain cycloperoxides, which emit light following decomposition. Given our understanding of the mechanism of chemiluminescence in those molecules, one would expect by extension that in the NO + O₃ reaction the chemiluminescent species (NO₂ in this case) is formed in the excited state through a reaction pathway that diverges from the ground state pathway near the transition state. A systematic search for such a pathway leads us to conclude that such a mechanism is unlikely. Instead, our study suggests that chemiluminescence in the NO + O₃ reaction is due to emission from the NO₂ vibronic states associated with the ground (X̃ ²A₁) and first excited (à ²B₂) electronic states, which are populated in the nascent NO₂ produced in the reaction. The vibronic coupling between the X̃ ²A₁ and à ²B₂ states of NO₂ is due to a conical intersection (CI), which is geometrically and energetically close to the à ²B₂ minimum energy geometry and only 1.3 eV higher than ground state NO₂. Further, the CI is 1.2 eV lower than the energy of the NO + O₃ reactants and therefore thermodynamically accessible following the reaction. An analysis of the product energy distribution indicates that the major fraction of the reaction energy is channeled into the vibrational modes of NO₂, sufficient to populate the vibronic states of NO₂ around the X̃/à CI. These vibronic states show dipole-allowed emission in a frequency range that is consistent with the observed broad chemiluminescence spectrum.

Ultrafast excited-state dynamics of 2,5-dimethylpyrrole

The ultrafast excited-state dynamics of 2,5-dimethylpyrrole is studied in detail following deep UV excitation.

Ultrafast excited-state dynamics of 2,4-dimethylpyrrole

Ultrafast excited-state dynamics of 2,4-dimethylpyrrole are studied in detail following deep UV excitation.

Bottom-up excited state dynamics of two cinnamate-based sunscreen filter molecules

We have used time-resolved pump–probe spectroscopy to explore E-MMC's and E-EHMC's excited state dynamics upon UV-B photoexcitation.