Femtochemistry of selected di-substituted benzenes

Fröbel, Sascha; Gilch, Peter

doi:10.1016/j.jphotochem.2015.05.036

Cited by 9 publications

(18 citation statements)

References 55 publications

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“…Figure shows another technologically relevant example of ESPT: the photoinduced release of ortho ‐nitrobenzyl‐related photolabile protecting groups. Gilch and co‐workers successfully applied FSRS to a variety of these compounds, including ortho ‐nitrobenzyl acetate (oNBAc), ortho ‐nitrotoluene, ortho ‐ethylnitrobenzene and ortho ‐nitrobenzaldehyde . These widely used systems for chemical synthesis show several competing de‐excitation pathways involving a cascade of elementary processes on various timescales.…”

Section: Systems and Applicationsmentioning

confidence: 99%

Femtosecond Stimulated Raman Spectroscopy

2016

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Femtosecond stimulated Raman spectroscopy (FSRS) is an ultrafast nonlinear optical technique that provides vibrational structural information with high temporal (sub-50 fs) precision and high spectral (10 cm(-1) ) resolution. Since the first full demonstration of its capabilities ≈15 years ago, FSRS has evolved into a mature technique, giving deep insights into chemical and biochemical reaction dynamics that would be inaccessible with any other technique. It is now being routinely applied to virtually all possible photochemical reactions and systems spanning from single molecules in solution to thin films, bulk crystals and macromolecular proteins. This review starts with an historic overview and discusses the theoretical and experimental concepts behind this technology. Emphasis is put on the current state-of-the-art experimental realization and several variations of FSRS that have been developed. The unique capabilities of FSRS are illustrated through a comprehensive presentation of experiments to date followed by prospects.

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Section: Systems and Applicationsmentioning

confidence: 99%

Femtosecond Stimulated Raman Spectroscopy

2016

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“…27 While these experiments provide key insights into the aci-rearrangement dynamics in neutrals, the solution-phase time scales may be affected by solvent interactions, and subsequent molecular dissociation pathways have not been explored. 27,28 Additionally, no aci-rearrangement or subsequent dissociation time scales have been reported in the 2-NT cation. As a result, knowledge of both the aci-rearrangement and subsequent dissociation mechanisms may be gained through femtosecond pump−probe measurements on the isolated 2-NT cation.…”

Section: Introductionmentioning

confidence: 99%

“…The ultrafast dynamics of the aci -rearrangement in neutral 2-NT and other 2-nitrobenzyl compounds have been studied in the solution phase using time-resolved pump–probe experiments with femtosecond laser pulses. , Upon excitation of 2-NT to the ππ* (S 2 ) state, aci -rearrangement occurs following relaxation to the nπ* (S 1 ) state on a time scale of 10 ps or after intersystem crossing to the triplet manifold on a time scale of 1300 ps . While these experiments provide key insights into the aci -rearrangement dynamics in neutrals, the solution-phase time scales may be affected by solvent interactions, and subsequent molecular dissociation pathways have not been explored. , Additionally, no aci -rearrangement or subsequent dissociation time scales have been reported in the 2-NT cation. As a result, knowledge of both the aci -rearrangement and subsequent dissociation mechanisms may be gained through femtosecond pump–probe measurements on the isolated 2-NT cation.…”

Section: Introductionmentioning

confidence: 99%

Conserved Vibrational Coherence in the Ultrafast Rearrangement of 2-Nitrotoluene Radical Cation

et al. 2019

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2-nitrotoluene (2-NT) is a good model for both photolabile protecting groups for organic synthesis and the military explosive 2,4,6-trinitrotoluene (TNT). In addition to the direct C−NO 2 bond-cleavage reaction that initiates detonation in TNT, 2-NT undergoes an H-atom attack reaction common to the photolabile 2-nitrobenzyl group, which forms the aci-nitro tautomer. In this work, femtosecond pump-probe measurements with mass spectrometric detection and density functional theory (DFT) calculations demonstrate that the initially-prepared vibrational coherence in 2-NT radical cation (2-NT +) is preserved following H-atom attack. Strong-field adiabatic ionization is used to prepare 2-NT + , which can overcome a modest 0.76 eV energy barrier to Hatom attack to form the aci-nitro tautomer as soon as ∼ 20−60 fs after ionization. Once formed, the aci-nitro tautomer spontaneously loses −OH to form C 7 H 6 NO + , which exhibits distinctly faster oscillations in its ion yield (290 fs period) as compared to the 2-NT + ion (380 fs period). The fast oscillations are attributed to the coherent torsional motion of the aci-nitro tautomer, which has a significantly faster computed torsional frequency (86.9 cm −1) than the 2-NT + ion (47.9 cm −1). Additional DFT calculations identify reaction pathways leading to the formation of the dissociation products C 7 H 6 NO + , C 7 H + 7 , and C 6 H 6 N +. Collectively, these results reveal a rich picture of coherently-and incoherently-driven dissociation pathways in 2-NT + .

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“…The hydrogen transfer leads to tautomerization and subsequent release of the protecting group resulting in the molecule being available in its active form . This ESIHT reaction in oNB and related moieties has been the subject of several experimental and theoretical studies, − but the precise mechanistic roles of the multiple close lying excited states of different spin multiplicities remains unknown.…”

Section: Introductionmentioning

confidence: 99%

“…The simplest oNB molecule demonstrating photoinduced tautomerization is o-nitrotoluene (oNT) and it therefore serves as a model to study the photochemistry of such caged compounds. 7,8,15,24,26 Recent time-resolved spectroscopic studies of oNT by Gilch and co-workers have shown that phototautomerization occurs on two distinctly different time scales, 1−10 and 1500 ps, with a quantum yield of 0.08. 15,26 The two time scales have been attributed to the singlet and triplet channels, respectively.…”

Section: Introductionmentioning

confidence: 99%

Intersystem Crossing Drives Photoisomerization in o-Nitrotoluene, a Model for Photolabile Caged Compounds

Gudem

Hazra

2018

J. Phys. Chem. A

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o-Nitrobenzyl (oNB) derivatives are widely used photolabile caged compounds in chemical and biological applications. The primary step in the photoinduced deprotection is an excited state intramolecular hydrogen transfer (ESIHT) leading to tautomerization of the oNB compound and subsequent release of the protecting group. The prototype molecule for studying such ESIHT is o-nitrotoluene (oNT), where hydrogen transfers from the methyl to the nitro group. Using the complete active space self-consistent field (CASSCF) method with second-order perturbative energy corrections (CASPT2), we have comprehensively investigated the photoisomerization and photo decay mechanisms in oNT. We have obtained the minimum energy crossing points (MECPs) between relevant electronic states and identified the singlet and triplet pathways. There is a barrierless path for oNT to relax to the lowest triplet state. In this T state, the ESIHT products are more stable than T oNT. Hydrogen-transfer occurs on the T state followed by relaxation to the ground state to give the isomerized product. A biradical intermediate proposed by previous studies is characterized to be the hydrogen-transferred T product. On the singlet pathway, in contrast to the triplet, the ground state tautomer is formed from the S oNT through a geometrically distant and energetically higher S/S conical intersection. Although nonadiabatic dynamical studies are essential for determining branching ratios, our study, which considers the accessibility of different MECPs based on geometry and energy, and the magnitude of spin-orbit coupling at singlet-triplet MECPs, suggests that a significant fraction of the isomerization yield is due to the triplet channel.

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Femtochemistry of selected di-substituted benzenes

Cited by 9 publications

References 55 publications

Femtosecond Stimulated Raman Spectroscopy

Femtosecond Stimulated Raman Spectroscopy

Conserved Vibrational Coherence in the Ultrafast Rearrangement of 2-Nitrotoluene Radical Cation

Intersystem Crossing Drives Photoisomerization in o-Nitrotoluene, a Model for Photolabile Caged Compounds

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