Whereas the photoinduced charge-transfer properties of electron donor-acceptor dyads are now well understood, those of symmetric conjugated architectures containing several identical donor-acceptor branches have started to be scrutinised much more recently. Here, we report on our investigation of the charge-transfer dynamics of a series of formally centrosymmetric triads consisting of a quadrupolar dihydropyrrolopyrrole core substituted with two identical diphenylethynyl lateral branches. Using a combination of time-resolved electronic and vibrational spectroscopies, we show that these molecules exhibit rich excited-state dynamics, which includes three different types of symmetry-breaking charge-transfer processes depending on the nature of the end substituents on the core and branches as well as on the solvent: (i) excited-state symmetry breaking within the core; (ii) charge transfer from the core to one of the two branches; (iii) charge transfer between the two branches.
View Article and Find Full Text PDFElectronic excitation in quadrupolar conjugated molecules rapidly localizes on a single electron donor-acceptor (DA) branch when in polar environments. The loss of center of inversion upon this excited-state symmetry breaking (ES-SB) can be monitored by exploiting the relaxation of the exclusion rules for IR and Raman vibrational transitions. Here, we compare ES-SB in a right-angled () and a centrosymmetric () DAD dyes using time-resolved IR spectroscopy.
View Article and Find Full Text PDFThe torsional disorder of conjugated dyes in the electronic ground state can lead to inhomogeneous broadening of the S ←S absorption band, allowing for the selective photoexcitation of molecules with different amounts of distortion. Here, we investigate how this affects electronic transitions to upper excited states. We show that torsion of a core-alkynylated push-pull dye can have opposite effects on the oscillator strength of its lowest-energy transitions.
View Article and Find Full Text PDFFluorescent flippers have been introduced as small-molecule probes to image membrane tension in living systems. This study describes the design, synthesis, spectroscopic and imaging properties of flippers that are elongated by one and two alkynes inserted between the push and the pull dithienothiophene domains. The resulting mechanophores combine characteristics of flippers, reporting on physical compression in the ground state, and molecular rotors, reporting on torsional motion in the excited state, to take their photophysics to new level of sophistication.
View Article and Find Full Text PDFThe inclusion of boron atoms into chiral π-conjugated systems is an effective strategy to unlock unique chiroptical properties. Herein, the preparation and characterization of a configurationally stable axially-chiral boramidine are reported, showcasing absorption in the UV domain, deep-blue fluorescence ( up to 94%), and |10| and values. Detailed photophysical studies and quantum-chemical calculations clearly elucidate the deactivation pathways of the emissive state to triplet excited states, involving increased spin-orbit coupling between the lowest singlet excited state and an upper triplet state.
View Article and Find Full Text PDFJ Phys Chem Lett
March 2024
So far, symmetry-breaking charge separation (SB-CS) has been observed with a limited number of chromophores and is usually inhibited by the formation of an excimer. We show here that thanks to fine-tuning of the interchromophore coupling via structural control, SB-CS can be operative with pyrene, despite its high propensity to form an excimer. This is realized with a bichromophoric system consisting of two pyrenes attached to a crown ether macrocycle, which can bind cations of different sizes.
View Article and Find Full Text PDFA large number of multipolar dyes undergo excited-state symmetry breaking (ESSB) in polar media. During this process, electronic excitation, initially distributed evenly over the molecule, localizes, at least partially, on one donor-acceptor branch. To resolve its initial stage, ESSB is investigated with a donor-acceptor-donor dye in binary mixtures of nonpolar and polar solvents using time-resolved infrared absorption spectroscopy.
View Article and Find Full Text PDFDye-sensitized photocatalytic systems (DSPs) have been extensively investigated for solar-driven hydrogen (H ) evolution. However, their application in carbon dioxide (CO ) reduction remains limited. Furthermore, current solar-driven CO -to-CO DSPs typically employ rhenium complexes as catalysts.
View Article and Find Full Text PDFElectron transfer (ET) quenching in nonpolar media is not as well understood as in polar environments. Here, we investigate the effect of dipole-dipole interactions between the reactants using ultrafast broadband electronic spectroscopy combined with molecular dynamics simulations. We find that the quenching of the S state of two polar dyes, coumarin 152a and Nile red, by the polar ,-dimethylaniline (DMA) in cyclohexane is faster by a factor up to 3 when exciting on the red edge rather than at the maximum of their S ← S absorption band.
View Article and Find Full Text PDFUnderstanding the origin of electron-phonon coupling in lead halide perovskites is key to interpreting and leveraging their optical and electronic properties. Here we show that photoexcitation drives a reduction of the lead-halide-lead bond angles, a result of deformation potential coupling to low-energy optical phonons. We accomplish this by performing femtosecond-resolved, optical-pump-electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in nanocrystals of FAPbBr.
View Article and Find Full Text PDFThere is a growing interest in developing dye-sensitized photocatalytic systems (DSPs) to produce molecular hydrogen (H ) as alternative energy source. To improve the sustainability of this technology, we replaced the sacrificial electron donor (SED), typically an expensive and polluting chemical, with an alcohol oxidation catalyst. This study demonstrates the first dye-sensitized system using a diketopyrrolopyrrole dye covalently linked to 2,2,6,6-tetramethyl-1-piperidine-N-oxyl (TEMPO) based catalyst for simultaneous H evolution and alcohol-to-aldehyde transformation operating in water with visible irradiation.
View Article and Find Full Text PDFPhotochem Photobiol Sci
January 2024
Photoinduced symmetry-breaking charge separation (SB-CS) results in the generation of charge carriers through electron transfer between two identical molecules, after photoexcitation of one of them. It is usually studied in systems where the two reacting moieties are covalently linked. Examples of photoinduced bimolecular SB-CS with organic molecules yielding free ions remain scarce due to solubility or aggregation issues at the high concentrations needed to study this diffusion-assisted process.
View Article and Find Full Text PDFA significant number of quadrupolar dyes behave as their dipolar analogues when photoexcited in polar environments. This is due to the occurrence of excited-state symmetry breaking (ES-SB), upon which the electronic excitation, initially distributed over the whole molecule, localises preferentially on one side. Here, we investigate the ES-SB properties of two A-D-A dyes, consisting of a pyrrolo-pyrrole donor (D) and either cyanophenyl or dicyanovinyl acceptors (A).
View Article and Find Full Text PDFThe nature of the lowest-energy electronic absorption band of crystal violet (CV) and particularly the origin of its high-energy shoulder have been debated since the middle of the past century. The most recent studies invoke a splitting of the S state upon symmetry breaking induced by interactions with the solvent and/or the counterion. Using a combination of stationary and time-resolved polarized spectroscopy together with quantum-chemical calculations, we show that torsional disorder in the ground-state results in an inhomogeneous broadening of the absorption band of CV.
View Article and Find Full Text PDFOccurrence of chiral recognition in bimolecular photoinduced electron transfer (ET) is difficult to identify because of the predominant role of diffusion. To circumvent this problem, we apply a combination of ultrafast time-resolved fluorescence and transient electronic absorption to look for stereoselectivity in the initial, static stage of ET quenching, where diffusion is not relevant. The fluorophore and electron acceptor is a cationic hexahelicene, whereas the quencher has either stereocentered (tryptophan) or axial (binaphthol) chirality.
View Article and Find Full Text PDFParent, unsubstituted porphycene and its two derivatives: 2,7,12,17-tetra--propylporphycene and 2,7,12,17-tetra--butylporphycene were substituted at the position with amino and nitro groups. These two families of porphycenes were characterized in detail with respect to their spectral, photophysical, and tautomeric properties. Two tautomers of similar energies coexist in the ground electronic state, but only one form dominates in the lowest excited singlet state.
View Article and Find Full Text PDFPhys Chem Chem Phys
November 2022
Conjugated molecules with phenylethynyl building blocks are usually characterised by torsional disorder at room temperature. They are much more rigid in the electronic excited state due to conjugation. As a consequence, the electronic absorption and emission spectra do not present a mirror-image relationship.
View Article and Find Full Text PDFThe excited-state properties of an amphiphilic porphyrin-fullerene dyad and of its porphyrin analogue adsorbed at the dodecane/water interface are investigated by using surface second-harmonic generation. Although the porphyrin is formally centrosymmetric, the second-harmonic spectra of both compounds are dominated by the intense Soret band of the porphyrin. Polarization-selective measurements and molecular dynamics simulations suggest an angle of about 45° between the donor-acceptor axis and the interfacial plane, with the porphyrin interacting mostly with the nonpolar phase.
View Article and Find Full Text PDFJ Phys Chem Lett
March 2022
The emissive properties of symmetric molecules containing several donor-acceptor branches are often similar to those of the single-branched analogues. This is due to the at least partial localization of the excitation on one branch. Detailed understanding of this excited-state symmetry breaking (ES-SB) requires the ability to monitor this process in real time.
View Article and Find Full Text PDFJ Phys Chem B
February 2022
The current developments in photoredox chemistry are stimulating a renewed interest for bimolecular photoinduced electron transfer reactions. Their investigation, initiated in the 1960s using conventional photochemical tools, resulted in a relatively simple reaction scheme. More recent studies, using not only spectroscopic techniques with better time resolution and extended spectral/temporal windows but also molecular dynamics simulations, reveal a more complex picture.
View Article and Find Full Text PDFThis article describes four fluorescent membrane tension probes that have been designed, synthesized, evaluated, commercialized and applied to current biology challenges in the context of the NCCR Chemical Biology. Their names are Flipper-TR, ER Flipper-TR, Lyso Flipper-TR, and Mito Flipper-TR. They are available from Spirochrome.
View Article and Find Full Text PDFPhys Chem Chem Phys
December 2021
The radical anion of 9,10-dicyanoanthracene (DCA) has been suggested to be a promising chromophore for photoredox chemistry, due to its nanosecond excited-state lifetime determined from indirect measurements. Here, we investigate the excited-state dynamics of the radical anion of three cyanoanthracenes, including DCA˙-, produced by photoinduced electron transfer in liquid using both pump-probe and pump-pump probe transient electronic absorption spectroscopy. All three excited radical ions are characterised by a 3-5 ps lifetime, due to efficient non-radiative deactivation to the ground state.
View Article and Find Full Text PDFThomas Bally has acquired international recognition for his work on the photochemistry of reactive intermediates, which include radical ions. Here, we present a brief overview of our investigations of the excited-state dynamics of radical ions in liquids at room temperature, which are still poorly documented. A better understanding of these dynamics is most relevant, as open-shell ions in the excited state are being increasingly used in redox photochemistry and have been proposed to play a key role in highly exergonic photoinduced electron transfer reactions.
View Article and Find Full Text PDFUnderstanding the distance dependence of the parameters underpinning Marcus theory is imperative when interpreting the results of experiments on electron transfer (ET). Unfortunately, most of these parameters are difficult or impossible to access directly with experiments, necessitating the use of computer simulations to model them. In this work, we use molecular dynamics simulations in conjunction with constrained density functional theory calculations to study the distance dependence of the electronic coupling matrix element, ||, for bimolecular ET.
View Article and Find Full Text PDF