scholarly journals Resolving photoisomerization dynamics via ultrafast UV-visible transient absorption spectroscopy

2021 ◽  
Author(s):  
◽  
Shyamal Prasad
Keyword(s):  
Absorption Spectroscopy ◽  
Ground State ◽  
Transient Absorption ◽  
Bond Cleavage ◽  
Ultra Violet ◽  
Solvent Polarity ◽  
Transient Absorption Spectroscopy ◽  
Bond Breakage ◽  
Solvent Dependence ◽  
Azobenzene Moiety

<p>Transient absorption spectroscopy has been employed to investigate three photo–active compounds; azobenzene, foldamer controlled by azobenzene, and oxazine. These compounds all have absorption in the ultra–violet regions responsible for their photo–active behavior. Due to this, the current transient absorption setup has been modified to extend the probing wavelength range to 320–650 nm, with the possibility of exciting the photo–active molecule in the ultra–violet.  Azobenzene is valuable in benchmarking and optimizing the transient absorption setup, it shows that the detection window has been extended out to 320 nm. By resolving the ground state bleach we have added support for the assignment of the final decay to thermalization in the ground state. Comparison of relaxation lifetime in acetonitrile and tetrahydrofuran shows no noticeable change in the photophysics of isomerization between the two solvents.  The foldamer family excited state relaxation is similar to azobenzene. There is an extension in the S₁ branching lifetime from 1.1 ps in azobenzene to 1.7 ps for foldamer 1 and 4.2 ps for foldamer 2. The separation of branching on the S₁ surface and relaxation through the S₁ to electronic ground state intersection was possible by comparison of azobenzene and foldamer family. The solvent effects show little difference for all members of the foldamer family expect for foldamer 2, suggesting that the dynamics of the azobenzene moiety are not affected by the larger macro–structure of the foldamer.  For oxazine it has been established, by varying solvent polarity, that isomerization happens through three states; bond breakage, transfer to a dark state, and the final photo–isomer. This is confirmed by further studies completed after the introduction of electron withdrawing fluorine atoms. Carbon–oxygen bond cleavage occurs on the picosecond timescale, with solvent dependent rotation occurring in hundreds of picoseconds. Fluorinated oxazine shows a strong solvent dependence with rotation suppressed for all but the most polar of solvents.</p>

scholarly journals Resolving photoisomerization dynamics via ultrafast UV-visible transient absorption spectroscopy

2021 ◽  
Author(s):  
◽  
Shyamal Prasad
Keyword(s):  
Absorption Spectroscopy ◽  
Ground State ◽  
Transient Absorption ◽  
Bond Cleavage ◽  
Ultra Violet ◽  
Solvent Polarity ◽  
Transient Absorption Spectroscopy ◽  
Bond Breakage ◽  
Solvent Dependence ◽  
Azobenzene Moiety

<p>Transient absorption spectroscopy has been employed to investigate three photo–active compounds; azobenzene, foldamer controlled by azobenzene, and oxazine. These compounds all have absorption in the ultra–violet regions responsible for their photo–active behavior. Due to this, the current transient absorption setup has been modified to extend the probing wavelength range to 320–650 nm, with the possibility of exciting the photo–active molecule in the ultra–violet.  Azobenzene is valuable in benchmarking and optimizing the transient absorption setup, it shows that the detection window has been extended out to 320 nm. By resolving the ground state bleach we have added support for the assignment of the final decay to thermalization in the ground state. Comparison of relaxation lifetime in acetonitrile and tetrahydrofuran shows no noticeable change in the photophysics of isomerization between the two solvents.  The foldamer family excited state relaxation is similar to azobenzene. There is an extension in the S₁ branching lifetime from 1.1 ps in azobenzene to 1.7 ps for foldamer 1 and 4.2 ps for foldamer 2. The separation of branching on the S₁ surface and relaxation through the S₁ to electronic ground state intersection was possible by comparison of azobenzene and foldamer family. The solvent effects show little difference for all members of the foldamer family expect for foldamer 2, suggesting that the dynamics of the azobenzene moiety are not affected by the larger macro–structure of the foldamer.  For oxazine it has been established, by varying solvent polarity, that isomerization happens through three states; bond breakage, transfer to a dark state, and the final photo–isomer. This is confirmed by further studies completed after the introduction of electron withdrawing fluorine atoms. Carbon–oxygen bond cleavage occurs on the picosecond timescale, with solvent dependent rotation occurring in hundreds of picoseconds. Fluorinated oxazine shows a strong solvent dependence with rotation suppressed for all but the most polar of solvents.</p>

scholarly journals UV-light induced vibrational coherences explain Kasha rule violation in frans-azobenzene

2019 ◽  
Vol 205 ◽  
pp. 09016
Author(s):  
Artur Nenov ◽  
Rocio Borrego-Varillas ◽  
Aurelio Oriana ◽  
Lucia Ganzer ◽  
Francesco Segatta ◽  
...  
Keyword(s):  
Absorption Spectroscopy ◽  
Ground State ◽  
Transient Absorption ◽  
Uv Light ◽  
Decay Channel ◽  
Transient Absorption Spectroscopy ◽  
Rule Violation

Sub-20-fs transient absorption spectroscopy and simulations show that CNN-bendings dominate the sub-ps dynamics of ππ*-excited trans-azobenzene, thereby driving the system to the ground state through a non-productive decay channel in violation of the Kasha rule.

Steady-state photochemistry (Pschorr cyclization) and nanosecond transient absorption spectroscopy of twisted 2-bromoaryl ketones

2011 ◽  
Vol 83 (4) ◽  
pp. 841-860 ◽  
Author(s):  
Jarugu Narasimha Moorthy ◽  
Subhas Samanta ◽  
Apurba L. Koner ◽  
Werner M. Nau
Keyword(s):  
Steady State ◽  
Absorption Spectroscopy ◽  
Transient Absorption ◽  
Bond Cleavage ◽  
Transient Absorption Spectroscopy ◽  
Polar Solvents ◽  
Transient Phenomena ◽  
X Ray ◽  
Aryl Radicals ◽  
Aryl Ketones

The steady-state as well as transient absorption spectroscopy of a series of 2-bromo-aryl ketones have been comprehensively examined to gain insights concerning (i) the transient phenomena (absorption spectral attributes as well as lifetimes), (ii) rates of C–Br homolysis, and (iii) the behavior of 2-aroylaryl radicals thus generated. The X-ray crystal structure analyses of selected ketones in which the mesomeric effects operate differently reveal that the two aryl rings are drastically twisted about the C=O bond. The twisting manifests itself in the spectral features of the transients, attributed to triplet–triplet (T–T) absorptions, such that they are not readily comparable in some cases to the transients of parent diaryl ketones that lack the 2-bromo group. By associating triplet decays with C–Br cleavage rates, the absolute rate data have been determined for diverse 2-bromoaryl ketones. With the exception of 2-bromo ketones containing meta-methoxy substituents, all other ketones are found to undergo efficient C–Br bond cleavage with rates of ca. 0.1–1.0 × 108 s–1. For m-methoxy-substituted ketones, intriguingly slower deactivation of the triplets was observed. Based on solvent-dependent variation of the lifetimes (longer lifetimes in polar solvents), intramolecular charge transfer has been proposed. The preparative photochemistry and transient phenomena permit invaluable inferences as to the reactivity of 2-aroylaryl radicals in general. Quantum yield determinations and product analyses reveal that highly electrophilic aryl radicals undergo radical recombination, in a poor hydrogen-donating solvent, almost exclusively (>90 %) in the absence of incentive for stabilization via conversion to π-conjugated hydrofluorenyl radicals. Of course, when the latter is feasible, Pschorr cyclization leads to productive photochemical outcome. Moderately electrophilic radicals that lack stabilization via conversion to hydrofluorenyl radicals lend themselves to intramolecular 1,5-hydrogen shifts in conjunction with the formation of dehalogenated diaryl ketones and cyclized fluorenones (Fls) or its analogs.

scholarly journals A strong steric hindrance effect on ground state, excited state, and charge separated state properties of a CuI-diimine complex captured by X-ray transient absorption spectroscopy

2014 ◽  
Vol 43 (47) ◽  
pp. 17615-17623 ◽  
Author(s):  
J. Huang ◽  
M. W. Mara ◽  
A. B. Stickrath ◽  
O. Kokhan ◽  
M. R. Harpham ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Excited State ◽  
Steady State ◽  
Absorption Spectroscopy ◽  
Steric Hindrance ◽  
Ground State ◽  
Transient Absorption ◽  
Transient Absorption Spectroscopy ◽  
Interfacial Electron Transfer ◽  
X Ray

Steady-state and transient structures of a copper diimine dye sensitizer on TiO2 nanoparticles undergoing photoinduced interfacial electron transfer are determined.

Excited-State Dynamics in the RNA Nucleotide Uridine 5’-Monophosphate Investigated by Femtosecond Broadband Transient Absorption Spectroscopy

2019 ◽  
Author(s):  
Matthew M. Brister ◽  
Carlos Crespo-Hernández
Keyword(s):  
Excited State ◽  
Excited States ◽  
Ground State ◽  
Transient Absorption ◽  
Electronic Energy ◽  
Transient Absorption Spectroscopy ◽  
Electronic Relaxation ◽  
Vibrationally Excited ◽  
Excited State Dynamics ◽  
Π State

<p></p><p> Damage to RNA from ultraviolet radiation induce chemical modifications to the nucleobases. Unraveling the excited states involved in these reactions is essential, but investigations aimed at understanding the electronic-energy relaxation pathways of the RNA nucleotide uridine 5’-monophosphate (UMP) have not received enough attention. In this Letter, the excited-state dynamics of UMP is investigated in aqueous solution. Excitation at 267 nm results in a trifurcation event that leads to the simultaneous population of the vibrationally-excited ground state, a longlived <sup>1</sup>n<sub>O</sub>π* state, and a receiver triplet state within 200 fs. The receiver state internally convert to the long-lived <sup>3</sup>ππ* state in an ultrafast time scale. The results elucidate the electronic relaxation pathways and clarify earlier transient absorption experiments performed for uracil derivatives in solution. This mechanistic information is important because long-lived nπ* and ππ* excited states of both singlet and triplet multiplicities are thought to lead to the formation of harmful photoproducts.</p><p></p>

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