Excited-State Symmetry Breaking and Localization in a Noncentrosymmetric Electron Donor-Acceptor-Donor Molecule.

Electronic 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 (1) and a centrosymmetric (2) DAD dyes using time-resolved IR spectroscopy. We show that the localization of the excitation can also be identified with the bent molecule 1. We find that contrary to dye 2, subpopulations with localized and delocalized excitation coexist for 1 in weak to medium polar solvents. This difference originates from the torsional disorder present in the excited state of 1 but not of 2. Additionally, irreversible localization in a bent molecule is shown to require higher solvent polarity than in a centrosymmetric one.

Excitation-Wavelength-Dependent Photophysics of a Torsionally Disordered Push-Pull Dye.

The torsional disorder of conjugated dyes in the electronic ground state can lead to inhomogeneous broadening of the S1 ←S0 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. Consequently, photoselection of planar and twisted molecules can be achieved by exciting in distinct absorption bands. Whereas this has limited effect in liquids due to fast planarization of the excited molecules, it strongly affects the overall photophysics in a polymeric environment, where torsional motion is hindered, allowing for the photoselection of molecules with different fluorescence quantum yields and intersystem-crossing dynamics.

Core-Alkynylated Fluorescent Flippers: Altered Ultrafast Photophysics to Track Thick Membranes.

Fluorescent 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. Intensity ratios in broadened excitation bands from differently twisted conformers of core-alkynylated flippers thus report on mechanical compression. Lifetime boosts from ultrafast excited-state planarization and lifetime drops from competitive intersystem crossing into triplet states report on viscosity. In standard lipid bilayer membranes, core-alkynylated flippers are too long for one leaflet and tilt or extend into disordered interleaflet space, which preserves rotor-like torsional disorder and thus weak, blue-shifted fluorescence. Flipper-like planarization occurs only in highly ordered membranes of matching leaflet thickness, where they light up and selectively report on these thick membranes with red-shifted, sharpened excitation maxima, high intensity and long lifetime.

Axially-chiral boramidine for detailed (chir)optical studies.

The 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 (Φ up to 94%), and ca. |10-3| gabs and glum 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.

Controlling Symmetry-Breaking Charge Separation in Pyrene Bichromophores.

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. By combining stationary and time-resolved spectroscopy together with molecular dynamics simulations, we demonstrate that the excited-state dynamics can be totally changed depending on the binding cation. Whereas strong coupling leads to rapid excimer formation, too weak coupling results in noninteracting chromophores. However, intermediate coupling, achieved upon binding of Mg2+, allows for SB-CS to be operative.

Excited-State Symmetry Breaking in Solvent Mixtures.

A 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. The presence of a few polar molecules around the dye is sufficient to initiate ESSB. Although the extent of asymmetry in a mixture is close to that in a pure solvent of similar polarity, the dynamics are slower and involve translational diffusion. However, preferential solvation in the mixtures leads to a larger local polarity. Furthermore, inhomogeneous broadening of the S1 ← S0 absorption band of the dye is observed in the mixtures, allowing for a photoselection of solutes with different local environments and ESSB dynamics.

Antenna Effect in Noble Metal-Free Dye-Sensitized Photocatalytic Systems Enhances CO2 -to-CO Conversion.

Dye-sensitized photocatalytic systems (DSPs) have been extensively investigated for solar-driven hydrogen (H2 ) evolution. However, their application in carbon dioxide (CO2 ) reduction remains limited. Furthermore, current solar-driven CO2 -to-CO DSPs typically employ rhenium complexes as catalysts. In this study, we have developed DSPs that incorporate noble metal-free components, specifically a zinc-porphyrin as photosensitizer (PS) and a cobalt-quaterpyridine as catalyst (CAT). Taking a significant stride forward, we have achieved an antenna effect for the first time in CO2 -to-CO DSPs by introducing a Bodipy as an additional chromophore to enhance light harvesting efficiency. The energy transfer from Bodipy to zinc porphyrin resulted in remarkable stability (turn over number (TON)=759 vs. CAT), and high CO evolution activity (42 mmol g-1 h-1 vs. CAT).

Dye-Sensitized Photocatalysis: Hydrogen Evolution and Alcohol-to-Aldehyde Oxidation without Sacrifical Electron Donor.

There is a growing interest in developing dye-sensitized photocatalytic systems (DSPs) to produce molecular hydrogen (H2 ) 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 H2 evolution and alcohol-to-aldehyde transformation operating in water with visible irradiation.

Photoinduced Electron Transfer between Dipolar Reactants: Solvent and Excitation Wavelength Effects.

Electron 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 S1 state of two polar dyes, coumarin 152a and Nile red, by the polar N,N-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 S1 ← S0 absorption band. This originates from the inhomogeneous broadening of the band due to a distribution of the number of quencher molecules around the dyes. As a consequence, red-edge excitation photoselects dyes in a DMA-rich environment. Such broadening is not present in acetonitrile, and no excitation wavelength dependence of the ET dynamics is observed. The quenching of both dyes is markedly faster in nonpolar than polar solvents, independently of the excitation wavelength. According to molecular dynamics simulations, this is due to the preferential solvation of the dyes by DMA in cyclohexane. The opposite preferential solvation is predicted in acetonitrile. Consequently, close contact between the reactants in acetonitrile requires partial desolvation. By contrast, the recombination of the quenching product is slower in nonpolar than in polar solvents and exhibits much smaller dependence, if any, on the excitation wavelength.

Coupling to octahedral tilts in halide perovskite nanocrystals induces phonon-mediated attractive interactions between excitons.

Understanding 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 FAPbBr3. Our results indicate a stronger coupling in FAPbBr3 than CsPbBr3. We attribute the enhanced coupling in FAPbBr3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multi-excitonic state constructively interfere, giving rise to a coupling strength that scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in lead halide perovskites.

Bimolecular photoinduced symmetry-breaking charge separation of perylene in solution.

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. Here we investigate the excited-state dynamics of perylene (Pe) at high concentrations in solvents of varying polarity. Transient absorption spectroscopy on the subnanosecond to microsecond timescales reveal that self-quenching of Pe in the lowest singlet excited state leads to excimer formation in all solvents used. Additionally, bimolecular SB-CS, resulting in the generation of free ions, occurs concurrently to excimer formation in polar media, with a relative efficiency that increases with the polarity of the solvent. Moreover, we show that SB-CS is most efficient in room-temperature ionic liquids due to a charge-shielding effect leading to a larger escape of ions and due to the high viscosity that disfavours excimer formation.

Excited-state symmetry breaking in quadrupolar pull-push-pull molecules: dicyanovinyl vs. cyanophenyl acceptors.

A 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). For this, we use time-resolved vibrational spectroscopy, comparing IR absorption and femtosecond stimulated Raman spectroscopies. Although dicyanovinyl is a stronger electron-withdrawing group, ES-SB is not observed with the dicyanovinyl-based dye even in highly polar media, whereas it already takes place in weakly polar solvents with dyes containing cyanophenyl accepting groups. This difference is attributed to the large electronic coupling between the D-A branches in the former dye, whose loss upon symmetry breaking cannot be counterbalanced by a gain in solvation energy. Comparison with analogues of the cyanophenyl-based dye containing different spacers reveals that interbranch coupling does not so much depend on the distance between the D-A subunits than on the nature of the spacer. We show that transient Raman spectra probe different modes of these centrosymmetric molecules but are consistent with the transient IR data. However, lifetime broadening of the Raman bands, probably due to the resonance enhancement, may limit the application of this technique for monitoring ES-SB.

Torsional Disorder, Symmetry Breaking, and the Crystal Violet Shoulder Controversy.

The 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 S1 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. The center of the band is mostly due to symmetric molecules with a degenerate S1 state, whereas the edges originate from transitions to the S1 and S2 states of distorted symmetry-broken molecules. Transient-absorption measurements with different excitation wavelengths reveal that these two groups of molecules interconvert rapidly in liquid but not in a rigid environment.

Looking for chiral recognition in photoinduced bimolecular electron transfer using ultrafast spectroscopy.

Occurrence 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. We found that, in all cases, the quenching dynamics are the same, within the limit of error, for different diastereomeric pairs in polar and medium-polar solvents. The same absence of chiral effect is observed for the recombination of the radical pair, which results from the quenching. Molecular dynamics simulations suggest that the distribution of inter-reactant distance is independent of the chirality of the acceptor and the donor. Close contact resulting in large electronic coupling is predicted to be possible with all diastereomeric pairs. In this case, ET is an adiabatic process, whose dynamics do no longer depend on the coupling, but are rather controlled by high-frequency intramolecular modes.

Spectroscopic investigation of photophysics and tautomerism of amino- and nitroporphycenes.

Parent, unsubstituted porphycene and its two derivatives: 2,7,12,17-tetra-n-propylporphycene and 2,7,12,17-tetra-t-butylporphycene were substituted at the meso 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 trans tautomers of similar energies coexist in the ground electronic state, but only one form dominates in the lowest excited singlet state. Absorption, magnetic circular dichroism (MCD), and emission anisotropy combined with quantum-chemical calculations led to the assignment of S1 and S2 transitions in both tautomers. Compared with the parent porphycene, the S1-S2 energy gap significantly increases; for one tautomeric form, the effect is twice as large as for the other. Both amino- and nitroporphycenes emit single fluorescence; previously reported dual emission of aminoporphycenes is attributed to a degradation product. Introduction of bulky t-butyl groups leads to a huge decrease in fluorescence intensity; this effect, arising from the interaction of the meso substituent with the adjacent t-butyl moiety, is particularly strong in the nitro derivative.

Torsional disorder and planarization dynamics: 9,10-bis(phenylethynyl)anthracene as a case study.

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. Here, we investigate how torsional disorder affects the excited state dynamics of 9,10-bis(phenylethynyl)anthracene in solvents of different viscosities and in polymers, using both stationary and ultrafast electronic spectroscopies. Temperature-dependent measurements reveal inhomogeneous broadening of the absorption spectrum at room temperature. This is confirmed by ultrafast spectroscopic measurements at different excitation wavelengths. Red-edge irradiation excites planar molecules that return to the ground state without significant structural dynamics. In this case, however, re-equilibration of the torsional disorder in the ground state can be observed. Higher-energy irradiation excites torsionally disordered molecules, which then planarise, leading to important spectral dynamics. The latter is found to occur partially via viscosity-independent inertial motion, whereas it is purely diffusive in the ground state. This dissimilarity is explained in terms of the steepness of the potential along the torsional coordinate.

Photoinduced Electron Transfer in a Porphyrin-Fullerene Dyad at a Liquid Interface.

The 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. Time-resolved measurements reveal a marked concentration dependence of the dynamics of both compounds upon Q-band excitation, indicating the occurrence of intermolecular quenching processes. The significant differences in dynamics and spectra between the dyad and the porphyrin analogue are explained by a self-quenching of the excited dyad via an intermolecular electron transfer.

Beyond the Threshold: A Study of Chalcogenophene-Based Two-Photon Initiators.

A series of nine soluble, symmetric chalcogenophenes bearing hexyl-substituted triphenylamines, indolocarbazoles, or phenylcarbazoles was designed and synthesized as potential two-photon absorption (2PA) initiators. A detailed photophysical analysis of these molecules revealed good 2PA properties of the series and, in particular, a strong influence of selenium on the 2PA cross sections, rendering these materials especially promising new 2PA photoinitiators. Structuring and threshold tests proved the efficiency and broad spectral versatility of two selenium-containing lead compounds as well as their applicability in an acrylate resin formulation. A comparison with commercial photoinitiators Irg369 and BAPO as well as sensitizer ITX showed that the newly designed selenium-based materials TPA-S and TPA-BBS outperform these traditional initiators by far both in terms of reactivity and dose. Moreover, by increasing the ultralow concentration of TPA-BBS, a further reduction of the polymerization threshold can be achieved, revealing the great potential of this series for application in two-photon polymerization (2PP) systems where only low laser power is available.

Watching Excited-State Symmetry Breaking in Multibranched Push-Pull Molecules.

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. Over the past few years, several spectroscopic approaches were shown to enable visualization of ES-SB and of its dynamics. They include the detection of new vibrational or electronic absorption bands associated with transitions that are forbidden in the symmetric excited state. Alternatively, ES-SB can be detected by observing transitions that become weaker or vanish upon localization of the excitation. Herein, we discuss these different approaches as well as their merits and weaknesses.

Elucidating the Mechanism of Bimolecular Photoinduced Electron Transfer Reactions.

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. This Perspective focuses on the results of these latest studies with neutral organic reactants, highlighting the time dependence of the quenching rate, the effect of mutual orientation of the reactants on the electronic coupling, and their consequence on the nature of the reaction product. Remaining questions, such as the occurrence of distant electron transfer in nonviscous liquids are also addressed, and possible directions toward their answer are proposed.