Publications of the Department of Physical Chemistry


Modelling IR Spectral Dynamics upon Symmetry Breaking of a Photo-Excited Quadrupolar Dye
Nazarov, A. E.; Ivanov, A. I.; Vauthey, E.
J. Phys. Chem. C 2020, in press.

A significant number of quadrupolar dyes with a D-π-A-π-D or A-π-D-π-A structure,where D and A are electron donor and acceptor groups, were shown to undergosymmetry breaking (SB) upon optical excitation. During this process, the electronicexcitation, originally distributed evenly over the molecule, concentrates on one D-π-Abranch, and the molecule becomes dipolar. This process can be monitored by timeresolved infra-red (TRIR) spectroscopy and causes significant spectral dynamics. Atheoretical model of excited-state SB developed earlier ( Ivanov, A. I.J. Phys. Chem.C,2018,122, 29165-29172) is extended to account for the temporal changes takingplace in the IR spectrum upon SB. This model can reproduce the IR spectral dynamicsobserved in the−C≡C−stretching region with a D-π-A-π-D dye in two polar solventsusing a single set of molecular parameters. This approach allows estimating the degreeof asymmetry of the excited state in different solvents as well as its change during SB.Additionally, the relative contribution of the different mechanisms responsible for thesplitting of the symmetric and antisymmetric−C≡C−stretching bands, which areboth IR active upon SB, can be determined.

Monitoring Fe(II) Spin-State Equilibria via Eu(III) Luminescence in Molecular Complexes: Dream or Reality?
Lathion, T.; Fürstenberg, A.; Besnard, C.; Hauser, A.; Bousseksou, A.; Piguet, C.
Inorg. Chem. 2020, in press.

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and noninvasive monitoring of magnetic information storage. In order to approach this goal at the molecular level, three segmental ligand strands, L4L6, were reacted with stoichiometric mixtures of divalent d-block cations (M(II) = Fe(II) or Zn(II)) and trivalent lanthanides (Ln(III) = La(III) or Eu(III)) in acetonitrile to give C3-symmetrical dinuclear triple-stranded helical [LnM(Lk)3]5+ cations, which can be crystallized with noncoordinating counter-anions. The divalent metal M(II) is six-coordinate in the pseudo-octahedral sites produced by the facial wrapping of the three didentate binding units, the ligand field of which induces variable Fe(II) spin-state properties in [LnFe(L4)3]5+ (strictly high-spin), [LnFe(L5)3]5+ (spin-crossover (SCO) around room temperature), and [LnFe(L6)3]5+ (SCO at very low temperature). The introduction of the photophysically active Eu(III) probe in [EuFe(Lk)3]5+ results in europium-centered luminescence modulated by variable intramolecular Eu(III) → Fe(II) energy-transfer processes. The kinetic analysis implies Eu(III) → Fe(II) quenching efficiencies close to 100% for the low-spin configuration and greater than 95% for the high-spin state. Consequently, the sensitivity of indirect luminescence detection of Fe(II) spin crossover is limited by the resulting weak Eu(III)-centered emission intensities, but the dependence of the luminescence on the temperature unambiguously demonstrates the potential of indirect lanthanide-based spin-state monitoring at the molecular scale.

Supporting Information (pdf, 1.5 MB)

Mechanosensitive Membrane Probes:  Push-Pull Papillons
Humeniuk, H. V.; Licari, G.; Vauthey, E.; Sakai, N.; Matile, S.
Supramol. Chem. 2020, in press.

Design, synthesis and evaluation of push-pull N,N′-diphenyl-dihydrodibenzo[a,c]phenazines are reported. Consistent with theoretical predictions, donors and acceptors attached to the bent mechanophore are shown to shift absorption maxima to either red or blue, depending on their positioning in the chromophore. Redshifted excitation of push-pull fluorophores is reflected in redshifted emission of both bent and planar excited states. The intensity ratios of the dual emission in more and less polar solvents imply that excited-state (ES) planarization decelerates with increasing fluorophore macrodipole, presumably due to attraction between the wings of closed papillons. ES planarization of highly polarisable papillons is not observed in lipid bilayer membranes. All push-pull papillon amphiphiles excel with aggregation-induced emission (AIE) from bent ES as micelles in water and mechanosensitivity in viscous solvents. They are not solvatochromic and only weakly fluorescent (QY < 4%).

Supplemental material (pdf)

Direct Access to Chiral Secondary Amides by Copper-Catalyzed Borylative Carboxamidation of Vinylarenes with Isocyanates
Fiorito, D.; Liu, Y.; Besnard, C.; Mazet, C.
J. Am. Chem. Soc. 2020, 142, archive unige:128621 pdf full text [restricted access]

A Cu-catalyzed borylative carboxamidation reaction has been developed using vinylarenes and isocyanates. Alkynes, branched 1,3-dienes and bicyclic alkenes were also found to be competent coupling partners. Using a chiral phosphan-amine ligand, an enantioselective variant of this transformation was developed, affording a set of α-chiral amides with unprecedented levels of enantioselectivity. The synthetic utility of the method was demonstrated through a series of representative stereoretentive post-catalytic derivatizations.

Supporting Information: Experimental procedures, characterization of all new compounds, spectroscopic, spectrometric, and X-ray data for compounds 3wa, (E)-5ab, and 3ja (CCDC 1914320–1914322) (PDF / 21.13 MB)
Crystallographic information files (CIF): #1 ( 276.58 kb) / #2 (730.19 kb) / #3 (405.19 kb)

Theory of fluorescence spectrum dynamics and its application to determining the relaxation characteristics of the solvent and intramolecular vibrations
Fedunov, R. G.; Yermolenko, I. P.; Nazarov, A. E.; Ivanov, A. I.; Rosspeintner, A.; Angulo, G.
J. Mol. Liq. 2020, in press.

A general analytical expression for the transient fluorescence spectrum is derived. The formation of a wave packet in the excited state of a fluorophore is described, assuming that the pump pulse has a Gaussian time-profile. The expression explicitly connects the relaxation characteristics of the medium with the spectral dynamics of a fluorophore. Fitting the expression to experimental spectral dynamics allows obtaining the solvent relaxation function. So far this approach was applicable for the analysis of experimental data when the pump pulse does not populate excited sublevels of intramolecular high-frequency vibrational modes. Here, the approach is generalized to include vibrational relaxation in the excited electronic state. In this case, fitting to the experimental spectral dynamics provides reliable information not only on the solvent relaxation, but also on the relaxation time constants of intramolecular high-frequency vibrational modes. This approach is applied to the excited state dynamics of coumarin 153 in multiple solvents, obtained from broadband fluorescence upconversion spectroscopy.

Untying the Photophysics of Quinolinium-Based Molecular Knots and Links
Caprice, K.; Aster, A.; Cougnon, F. B. L.; Kumpulainen, T.
Chem. Eur. J. 2020, in press.

Today, complex molecular knots and links are still difficult to synthesize and the properties arising from their topology are mostly unknown. Here, we report on a comparative photophysical study carried out on a family of closely related quinolinium-based knots and links to determine the impact exerted by topology on the molecular backbone. Our results indicate that topology has a negligible influence on the behavior of loosely braided molecules, which mostly behave like their unbraided equivalents. On the other hand, tightly braided molecules display distinct features. Their higher packing density results in a pronounced ability to resist deformation, a significant reduction in the solvent-accessible surface area and favors close-range π-π interactions between the quinolinium-units and neighboring aromatics. Finally, the sharp alteration in behavior between loosely and tightly braided molecules sheds light upon the factors contributing to braiding tightness.

Supporting Information (pdf / 8 MB)