Resolving photoisomerization dynamics via ultrafast UV-visible transient absorption spectroscopy

<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>

Resolving photoisomerization dynamics via ultrafast UV-visible transient absorption spectroscopy

<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>

Structural Dynamics of C2F4I2 in Cyclohexane Studied via Time-Resolved X-ray Liquidography

The halogen elimination of 1,2-diiodoethane (C2H4I2) and 1,2-diiodotetrafluoroethane (C2F4I2) serves as a model reaction for investigating the influence of fluorination on reaction dynamics and solute–solvent interactions in solution-phase reactions. While the kinetics and reaction pathways of the halogen elimination reaction of C2H4I2 were reported to vary substantially depending on the solvent, the solvent effects on the photodissociation of C2F4I2 remain to be explored, as its reaction dynamics have only been studied in methanol. Here, to investigate the solvent dependence, we conducted a time-resolved X-ray liquidography (TRXL) experiment on C2F4I2 in cyclohexane. The data revealed that (ⅰ) the solvent dependence of the photoreaction of C2F4I2 is not as strong as that observed for C2H4I2, and (ⅱ) the nongeminate recombination leading to the formation of I2 is slower in cyclohexane than in methanol. We also show that the molecular structures of the relevant species determined from the structural analysis of TRXL data provide an excellent benchmark for DFT calculations, especially for investigating the relevance of exchange-correlation functionals used for the structural optimization of haloalkanes. This study demonstrates that TRXL is a powerful technique to study solvent dependence in the solution phase.

Striking Solvent Dependence of Total Emission and Circularly Polarised Luminescence in Coordinatively Saturated Chiral Europium Complexes: Solvation Significantly Perturbs the Ligand Field

Examination of total emission and circularly polarised luminescence (CPL) spectra of three 9-coordinate Eu(III) complexes with well-defined speciation shows that the ligand fields of these C3 symmetric complexes are extremely...

Solvent Dependence of Structural Dynamics and Spin-flip Processes in 3,4,5-tri(9H-carbazole-9-yl)benzonitrile (ortho-3CzBN)

We have investigated the solvent-dependence of structural changes along with intersystem crossing of a thermally activated delayed fluorescence (TADF) molecule, 3,4,5-tri(9H-carbazole-9-yl)benzonitrile (o-3CzBN), in toluene, tetrahydrofuran, and acetonitrile solutions using time-resolved infrared (TR-IR) spectroscopy and DFT calculations. We found that the geometries of the S1 and T1 states are very similar in all solvents though the photophysical properties mostly depend on the solvent. In addition, the time-dependent DFT calculations based on these geometries suggested that the thermally activated delayed fluorescence process of o-3CzBN is governed more by the higher-lying excited states than by the structural changes in the excited states.<br>